Package Exports

@jetio/schema-builder

Readme

📐 Schema Builder Guide

The Schema Builder provides a fluent, type-safe API for constructing JSON Schemas programmatically. Build complex schemas with autocomplete, validation, and zero boilerplate and get automatic type inference.

Note: This package includes @jetio/validator as a dependency. You get both the builder AND the fastest JSON Schema validator in one install.

Important: Jetio/schema-builder is a JSON Schema spec-compliant tool built on top of @jetio/validator. To utilize it to its fullest potential, it's essential to understand the main validator package. All core documentation about validation rules, error handling, $data references, and advanced features can be found in the Validator Documentation.

📦 Installation

npm install @jetio/schema-builder
# or
yarn add @jetio/schema-builder
# or
pnpm add @jetio/schema-builder

import { SchemaBuilder, RefBuilder } from "@jetio/schema-builder";
import { JetValidator } from "@jetio/schema-builder"; // Re-exported from @jetio/validator

Just need the validator? Install @jetio/validator directly for a smaller bundle.

Type Inference

JetIO Schema Builder includes Json Schema spec compliant automatic TypeScript type inference through Jet.Infer<>. Write your schema once and get both runtime validation AND compile-time types!

import { SchemaBuilder, Jet } from "@jetio/schema-builder";

const userSchema = new SchemaBuilder()
  .object()
  .properties({
    id: (s) => s.number(),
    name: (s) => s.string(),
    email: (s) => s.string().format('email')
  })
  .required(['id', 'name', 'email'])
  .build();

// Automatically infer TypeScript type from schema
type User = Jet.Infer<typeof userSchema>;
/*
{
  id: number;
  name: string;
  email: string;
}
*/

// Type-safe usage
const user: User = {
  id: 1,
  name: "Alice",
  email: "alice@example.com"
}; // ✅

const invalidUser: User = {
  id: 1,
  name: "Bob"
  // ❌ TypeScript Error: Property 'email' is missing
};

Why Type Inference Matters

Single Source of Truth: Schema and types never drift out of sync
Automatic Updates: Change the schema, types update automatically
Runtime + Compile Time Safety: Catch errors at both stages
Zero Duplication: No need to write interfaces separately

Advanced Type Inference

Type inference works with all schema features:

// Discriminated unions
const shapeSchema = new SchemaBuilder()
  .oneOf(
    (s) => s.object()
      .properties({
        type: (s) => s.const('circle'),
        radius: (s) => s.number()
      })
      .required(['type', 'radius']),
    (s) => s.object()
      .properties({
        type: (s) => s.const('rectangle'),
        width: (s) => s.number(),
        height: (s) => s.number()
      })
      .required(['type', 'width', 'height'])
  )
  .build();

type Shape = Jet.Infer<shapeSchema>;
// { type: 'circle'; radius: number } | { type: 'rectangle'; width: number; height: number }

// Conditionals with elseIf
const accountSchema = new SchemaBuilder()
  .object()
  .properties({
    accountType: (s) => s.string(),
    username: (s) => s.string(),
    companyName: (s) => s.string()
  })
  .required(['accountType'])
  .if((s) => s.object().properties({ accountType: (s) => s.const('personal') }))
  .then((s) => s.object().required(['username']))
  .elseIf((s) => s.object().properties({ accountType: (s) => s.const('business') }))
  .then((s) => s.object().required(['companyName']))
  .end()
  .build();

type Account = Jet.Infer<accountSchema>;
// TypeScript understands the conditional branches!

📖 For complete type inference documentation, see Type Inference Guide

Topics covered in the Type Inference guide:

Primitives, objects, arrays, and their type inference
Pattern properties with template literal types
Multiple types and union inference
Discriminated unions with oneOf/anyOf
Conditional type inference (if/then/else/elseIf)
Complex compositions with allOf
Required vs optional property splitting
Type inference limitations and workarounds -addtionalItems/Properties, unevaluatedProperties/Items, patternProperties.

🚀 Try It Live

Using with @jetio/validator

import { SchemaBuilder, Jet, JetValidator } from "@jetio/schema-builder";

const accountSchema = new SchemaBuilder()
  .object()
  .properties({
    accountType: (s) => s.string(),
    username: (s) => s.string(),
    companyName: (s) => s.string(),
    email: (s) => s.string().format("email"),
  })
  .required(["accountType", "email"])
  .if((s) =>
    s.object().properties({
      accountType: (s) => s.const("personal"),
    })
  )
  .then((s) => s.object().required(["username"]))
  .elseIf((s) =>
    s.object().properties({
      accountType: (s) => s.const("business"),
    })
  )
  .then((s) => s.object().required(["companyName"]))
  .end()
  .build();

// 1. See the generated JSON Schema
console.log(JSON.stringify(accountSchema, null, 2));
// {
//   "type": "object",
//   "properties": {
//     "accountType": { "type": "string" },
//     "username": { "type": "string" },
//     "companyName": { "type": "string" },
//     "email": { "type": "string", "format": "email" }
//   },
//   "required": ["accountType", "email"],
//   "if": { "properties": { "accountType": { "const": "personal" } } },
//   "then": { "required": ["username"] },
//   "elseIf": [
//     {
//       "if": { "properties": { "accountType": { "const": "business" } } },
//       "then": { "required": ["companyName"] }
//     }
//   ]
// }

// 2. Type inference
type Account = Jet.Infer<typeof accountSchema>;
// {
//   accountType: "personal";
//   email: string;
//   username: string;
//   companyName?: string;
// } | {
//   accountType: "business";
//   email: string;
//   companyName: string;
//   username?: string;
// } | {
//   accountType: string;
//   email: string;
//   username?: string;
//   companyName?: string;
// }

// 3. Runtime validation
const validator = new JetValidator({ allErrors: true });
const validate = validator.compile(accountSchema);

console.log(validate({
  accountType: "personal",
  email: "alice@example.com",
  username: "alice"
})); // true

console.log(validate({
  accountType: "personal",
  email: "alice@example.com"
  // missing username!
})); // false

console.log(validate.errors);
// [{ dataPath: '/', keyword: 'required', message: "must have required property 'username'" }]

🎯 Quick Start

import { SchemaBuilder } from "@jetio/schema-builder";

// Simple schema
const userSchema = new SchemaBuilder()
  .object()
  .properties({
    name: s => s.string().minLength(2),
    age: s => s.number().minimum(18)
  })
  .required(['name', 'age'])
  .build();

// Equivalent to:
{
  type: "object",
  properties: {
    name: { type: "string", minLength: 2 },
    age: { type: "number", minimum: 18 }
  },
  required: ["name", "age"]
}

Draft Support

The Schema Builder supports JSON Schema keywords from Draft 06 through Draft 2020-12, including:

Draft 06/07: definitions, dependencies, items (array form), additionalItems
Draft 2019-09: $defs, dependentSchemas, dependentRequired, unevaluatedProperties, unevaluatedItems
Draft 2020-12: prefixItems, $dynamicRef, $dynamicAnchor
Extension: elseIf (Jet-Validator extension for cleaner conditionals)

Use the appropriate keywords for your target draft version. The builder doesn't enforce draft restrictions — you can mix keywords, but ensure your validator supports them.

The builder follows json schema rules which means a schema can either be a standard json schema or a boolean.

Basic Types

Simple Type Declarations

// String
const nameSchema = new SchemaBuilder().string().build();
// { type: "string" }

// Number
const ageSchema = new SchemaBuilder().number().build();
// { type: "number" }

// Integer
const countSchema = new SchemaBuilder().integer().build();
// { type: "integer" }

// Boolean
const activeSchema = new SchemaBuilder().boolean().build();
// { type: "boolean" }

// Null
const nullSchema = new SchemaBuilder().null().build();
// { type: "null" }

Multiple Types

// Chain type methods to create type arrays
const flexibleSchema = new SchemaBuilder().string().number().build();
// { type: ["string", "number"] }

// Nullable values
const nullableString = new SchemaBuilder().string().null().minLength(5).build();
// { type: ["string", "null"], minLength: 5 }

Universal Keywords

These work with any type and are not locked to a specific type builder:

const schema = new SchemaBuilder()
  .$schema("https://json-schema.org/draft/2020-12/schema")
  .$id("https://json-schema.org/draft/2020-12/schema")
  .title("User Email")
  .description("The user's primary email address")
  .default("user@example.com")
  .examples("alice@example.com", "bob@example.com")
  .enum(["alice@example.com", "bob@example.com", "admin@example.com"])
  .readOnly()
  .build();

// Result:
{
  $schema: "https://json-schema.org/draft/2020-12/schema",
  $id: "https://json-schema.org/draft/2020-12/schema"
  title: "User Email",
  description: "The user's primary email address",
  default: "user@example.com",
  examples: ["alice@example.com", "bob@example.com"],
  enum: ["alice@example.com", "bob@example.com", "admin@example.com"],
  readOnly: true
}

Universal keywords include:

Keyword	Description
`.$schema(schema)`	Schema draft
`.$id(schemaId)`	Unique schema identifier
`.$ref(ref)`	Schema reference
`.$dynamicRef(dynamicRef)`	Dynamic schema reference
`.$anchor(anchor)`	Schema identifier
`.$dynamicAnchor(dynamicAnchor)`	Dynamic schema identifier
`.definitions(definitions)`	Schema definitions(draft 7 and below)
`.$defs(defs)`	Schema definitions
`.title(title)`	Human-readable title
`.description(desc)`	Human-readable description
`.default(value)`	Default value
`.examples(...values)`	Example values
`.enum(values)`	Array of values (any type)
`.const(value)`	Single allowed value (any type)
`.readOnly()`	Mark as read-only
`.writeOnly()`	Mark as write-only
`.not(schema)`	Negation
`.anyOf(...schemas)`	Match at least one
`.allOf(...schemas)`	Match all
`.oneOf(...schemas)`	Match exactly one
`.if(condition)`	Conditional validation
`.errorMessage(msg)`	Custom error messages
`.option(key, value)`	Any custom keyword

These can be combined with any type:

// Enum with numbers
new SchemaBuilder().number().enum([1, 2, 3, 5, 8, 13]).build();

// Const with boolean
new SchemaBuilder().boolean().const(true).build();

// Composition without a type
new SchemaBuilder()
  .anyOf(
    (s) => s.string(),
    (s) => s.number(),
  )
  .build();

TypeScript Type Locking

When chaining type methods, the first type called determines which keywords TypeScript makes available:

// String-first: string keywords available
new SchemaBuilder()
  .string() // Locks to StringSchemaBuilder
  .number() // Adds "number" to type array and changes to NumberSchemaBuilder
  .minLength(5) // ❌ TypeScript error - String keyword
  .pattern(/^a/) // ❌ TypeScript error - String keyword
  .minimum(0); // ✅ number keyword - available

// Number-first: number keywords available
new SchemaBuilder()
  .number() // Locks to NumberSchemaBuilder
  .string() // Adds "string" to type array, changed to StringSchemaBuilder
  .minimum(0) // ❌ TypeScript error - Number keyword
  .multipleOf(5) // ❌ TypeScript error - Number keyword
  .minLength(5); // ✅ string keyword - available

// Correct Approach
new SchemaBuilder()
  .number()
  .minimum(0)
  .multipleOf(5)
  .string()
  .minLength(5);

Why this design?

This ensures strict typing, making sure only methods of the desired type are accessible. The last type called decides the available methods, but users can still accept other types if they want a multi-type schema with one primary type's constraints.

If you need to define constraints for multiple types, just call the new type after the constraints for existing type is fully defined :

const schema = new SchemaBuilder()
  .string()
  .minLength(5) // String constraint
  .number() // Branches to  NumberSchemaBuilder
  .minimum(0) // Number constraint now available
  .build();

// Result: { type: ["string", "number"], minLength: 5, minimum: 0 }

String Schemas

Length Constraints

const usernameSchema = new SchemaBuilder()
  .string()
  .minLength(3)
  .maxLength(20)
  .build();
// { type: "string", minLength: 3, maxLength: 20 }

Pattern Matching

// Using RegExp
const slugSchema = new SchemaBuilder()
  .string()
  .pattern(/^[a-z0-9-]+$/)
  .build();

// Using string
const emailPattern = new SchemaBuilder()
  .string()
  .pattern("^[\\w-\\.]+@([\\w-]+\\.)+[\\w-]{2,4}$")
  .build();

Format Validation

const contactSchema = new SchemaBuilder()
  .object()
  .properties({
    email: (s) => s.string().format("email"),
    website: (s) => s.string().format("uri"),
    createdAt: (s) => s.string().format("date-time"),
  })
  .build();

// Supported formats: email, uri, uri-reference, uri-template,
// date, time, date-time, duration, uuid, ipv4, ipv6, hostname,
// json-pointer, relative-json-pointer, regex

Number Schemas

Numeric Constraints

const priceSchema = new SchemaBuilder()
  .number()
  .minimum(0)
  .maximum(1000000)
  .build();
// { type: "number", minimum: 0, maximum: 1000000 }

// Exclusive bounds
const percentageSchema = new SchemaBuilder()
  .number()
  .exclusiveMinimum(0)
  .exclusiveMaximum(100)
  .build();
// { type: "number", exclusiveMinimum: 0, exclusiveMaximum: 100 }

Multiple Of

const evenNumberSchema = new SchemaBuilder().integer().multipleOf(2).build();

const priceInCents = new SchemaBuilder().number().multipleOf(0.01).build();

Convenience Methods

// Positive numbers (>= 0)
const positiveSchema = new SchemaBuilder().number().positive().build();
// { type: "number", minimum: 0 }

// Negative numbers (<= 0)
const negativeSchema = new SchemaBuilder().number().negative().build();
// { type: "number", maximum: 0 }

// Range shorthand
const scoreSchema = new SchemaBuilder().number().range(0, 100).build();
// { type: "number", minimum: 0, maximum: 100 }

Enums & Constants

// Enum (multiple allowed values)
const statusSchema = new SchemaBuilder()
  .string()
  .enum(["active", "inactive", "pending"])
  .build();

// Const (single allowed value)
const typeSchema = new SchemaBuilder().string().const("user").build();

Array Schemas

Basic Arrays

// Array with item schema
const numbersSchema = new SchemaBuilder()
  .array()
  .items((s) => s.number())
  .build();
// { type: "array", items: { type: "number" } }

// With size constraints
const tagsSchema = new SchemaBuilder()
  .array()
  .items((s) => s.string())
  .minItems(1)
  .maxItems(10)
  .uniqueItems(true)
  .build();

Tuple Validation (Prefix Items)

// Fixed-length arrays with different types per position
const coordinateSchema = new SchemaBuilder()
  .array()
  .prefixItems(
    s => s.number(), // latitude
    s => s.number()  // longitude
  )
  .minItems(2)
  .maxItems(2)
  .build();

// Result:
{
  type: "array",
  prefixItems: [
    { type: "number" },
    { type: "number" }
  ],
  minItems: 2,
  maxItems: 2
}

Contains & Count Constraints

// Array must contain at least one number > 100
const arrayWithLargeNumber = new SchemaBuilder()
  .array()
  .items((s) => s.number())
  .contains((s) => s.number().minimum(100))
  .minContains(1)
  .maxContains(5)
  .build();

Complex Array Items

const usersSchema = new SchemaBuilder()
  .array()
  .items((s) =>
    s
      .object()
      .properties({
        id: (s) => s.integer(),
        name: (s) => s.string().minLength(1),
        email: (s) => s.string().format("email"),
        roles: (s) => s.array().items((s) => s.string()),
      })
      .required(["id", "name", "email"]),
  )
  .minItems(1)
  .build();

Unevaluated Items (Draft 2019-09+)

Control validation of array items not covered by prefixItems, items, or contains:

// Disallow any unevaluated items
const strictTuple = new SchemaBuilder()
  .array()
  .prefixItems(
    s => s.string(),
    s => s.number()
  )
  .unevaluatedItems(false)
  .build();

// Result:
{
  type: "array",
  prefixItems: [
    { type: "string" },
    { type: "number" }
  ],
  unevaluatedItems: false
}
// Valid: ["hello", 42]
// Invalid: ["hello", 42, "extra"]

// Allow unevaluated items with schema
const flexibleTuple = new SchemaBuilder()
  .array()
  .prefixItems(
    s => s.string(),
    s => s.number()
  )
  .unevaluatedItems(s => s.boolean())
  .build();

// Valid: ["hello", 42, true, false]
// Invalid: ["hello", 42, "not a boolean"]

// Allow any unevaluated items
const openTuple = new SchemaBuilder()
  .array()
  .prefixItems(
    s => s.string(),
    s => s.number()
  )
  .unevaluatedItems(true)
  .build();

// Valid: ["hello", 42, anything, anything, ...]

Additional Items (Draft 07 and earlier)

For older drafts, use additionalItems with array-form items:

// Disallow additional items beyond the tuple
const strictTupleLegacy = new SchemaBuilder()
  .array()
  .items(
    { type: "string" },
    { type: "number" }
  )
  .additionalItems(false)
  .build();

// Result:
{
  type: "array",
  items: [
    { type: "string" },
    { type: "number" }
  ],
  additionalItems: false
}
// Valid: ["hello", 42]
// Invalid: ["hello", 42, "extra"]

// Allow additional items with schema
const flexibleTupleLegacy = new SchemaBuilder()
  .array()
  .items(
    s => s.string(),
    s => s.number()
  )
  .additionalItems(s => s.boolean())
  .build();

// Valid: ["hello", 42, true, false]
// Invalid: ["hello", 42, "not a boolean"]

// Allow any additional items
const openTupleLegacy = new SchemaBuilder()
  .array()
  .items(
    s => s.string(),
    s => s.number()
  )
  .additionalItems(true)
  .build();

// Valid: ["hello", 42, anything, ...]

Note: In Draft 2020-12, array-form items was replaced by prefixItems, and additionalItems was replaced by items (schema form) for items beyond the tuple. Use unevaluatedItems for items not validated by any keyword.

tuple prefixItems and items was changed from .prefixItems([]) and .items([]) to .prefixItems(...[]) and .items(...[])

Object Schemas

Properties And Required

// Basic object
const personSchema = new SchemaBuilder()
  .object()
  .properties({
    firstName: (s) => s.string(),
    lastName: (s) => s.string(),
    age: (s) => s.integer().minimum(0),
  })
  .required(["firstName", "lastName"])
  .build();

Pattern Properties

// Properties matching a pattern
const configSchema = new SchemaBuilder()
  .object()
  .properties({
    version: (s) => s.string(),
  })
  .patternProperties({
    "^env_": (s) => s.string(), // env_* properties must be strings
    "^flag_": (s) => s.boolean(), // flag_* properties must be booleans
  })
  .build();

// Matches: { version: "1.0", env_mode: "production", flag_debug: true }

Additional Properties

// Disallow additional properties (strict)
const strictSchema = new SchemaBuilder()
  .object()
  .properties({
    name: (s) => s.string(),
  })
  .additionalProperties(false)
  .build();

// Allow additional properties with schema
const flexibleSchema = new SchemaBuilder()
  .object()
  .properties({
    knownProp: (s) => s.string(),
  })
  .additionalProperties((s) => s.number())
  .build();
// Any additional property must be a number

Unevaluated Properties (Draft 2019-09+)

Control validation of properties not evaluated by properties, patternProperties, additionalProperties, or any subschema in allOf, anyOf, oneOf, if/then/else:

// Disallow unevaluated properties
const strictComposition = new SchemaBuilder()
  .object()
  .allOf(
    (s) =>
      s.object().properties({
        name: (s) => s.string(),
      }),
    (s) =>
      s.object().properties({
        age: (s) => s.integer(),
      }),
  )
  .unevaluatedProperties(false)
  .build();

// Valid: { name: "Alice", age: 30 }
// Invalid: { name: "Alice", age: 30, extra: "oops" }

// Allow unevaluated properties with schema
const flexibleComposition = new SchemaBuilder()
  .object()
  .allOf((s) =>
    s.object().properties({
      id: (s) => s.integer(),
    }),
  )
  .unevaluatedProperties((s) => s.string())
  .build();

// Valid: { id: 1, name: "Alice", description: "A user" }
// Invalid: { id: 1, count: 42 } // count is number, not string

// Allow any unevaluated properties
const openComposition = new SchemaBuilder()
  .object()
  .allOf((s) =>
    s.object().properties({
      id: (s) => s.integer(),
    }),
  )
  .unevaluatedProperties(true)
  .build();

// Valid: { id: 1, anything: "goes" }

Property Name Constraints

// Property names must match a pattern
const schema = new SchemaBuilder()
  .object()
  .propertyNames((s) => s.string().pattern("^[a-z_]+$"))
  .build();
// All property names must be lowercase with underscores

Property Count

const schema = new SchemaBuilder()
  .object()
  .minProperties(1)
  .maxProperties(10)
  .build();

Dependent Schemas

// When 'creditCard' exists, require 'billingAddress'
const paymentSchema = new SchemaBuilder()
  .object()
  .properties({
    creditCard: (s) => s.string(),
    billingAddress: (s) => s.string(),
    shippingAddress: (s) => s.string(),
  })
  .dependentSchemas({
    creditCard: (s) => s.object().required(["billingAddress"]),
  })
  .build();

Dependent Required

// When 'country' is present, 'state' must also be present
const addressSchema = new SchemaBuilder()
  .object()
  .properties({
    street: (s) => s.string(),
    city: (s) => s.string(),
    state: (s) => s.string(),
    country: (s) => s.string(),
  })
  .dependentRequired({
    country: ["state"],
    state: ["city"],
  })
  .build();

Dependencies

// When 'creditCard' exists, require 'billingAddress'
const paymentSchema = new SchemaBuilder()
  .object()
  .properties({
    creditCard: (s) => s.string(),
    billingAddress: (s) => s.string(),
    shippingAddress: (s) => s.string(),
    street: (s) => s.string(),
    city: (s) => s.string(),
    state: (s) => s.string(),
    country: (s) => s.string(),
  })
  .dependencies({
    creditCard: (s) => s.object().required(["billingAddress"]),
    country: ["state"],
    state: ["city"],
  })
  .build();

Nested Objects

const companySchema = new SchemaBuilder()
  .object()
  .properties({
    name: (s) => s.string(),
    address: (s) =>
      s
        .object()
        .properties({
          street: (s) => s.string(),
          city: (s) => s.string(),
          country: (s) => s.string(),
        })
        .required(["street", "city", "country"]),
    employees: (s) =>
      s.array().items((s) =>
        s.object().properties({
          name: (s) => s.string(),
          role: (s) => s.string(),
        }),
      ),
  })
  .required(["name", "address"])
  .build();

Removing Properties

When extending schemas, remove unwanted fields:

const fullSchema = new SchemaBuilder()
  .object()
  .properties({
    id: (s) => s.integer(),
    name: (s) => s.string(),
    password: (s) => s.string(),
    secret: (s) => s.string(),
  })
  .required(["id", "name", "password"]);

const publicSchema = fullSchema
  .extend()
  .remove(
    ["password", "secret"], // Fields to remove
    ["properties", "required"], // Remove from both places
  )
  .build();

// Available targets: "properties", "required", "patternProperties",
//                    "dependencies", "dependentRequired"

Making Fields Optional

Remove all required constraints:

const strictSchema = new SchemaBuilder()
  .object()
  .properties({ name: (s) => s.string() })
  .required(["name"]);

const looseSchema = strictSchema
  .extend()
  .optional() // Removes required array entirely
  .build();

Boolean Schema Values

Many keywords accept true, false, a schema object, or a builder callback. Boolean values have special meanings:

Object Keywords

// additionalProperties
.additionalProperties(false)          // No extra properties allowed
.additionalProperties(true)           // Any extra properties allowed (default)
.additionalProperties(s => s.string()) // Extra properties must be strings

// unevaluatedProperties
.unevaluatedProperties(false)         // No unevaluated properties allowed
.unevaluatedProperties(true)          // Any unevaluated properties allowed
.unevaluatedProperties(s => s.number()) // Unevaluated must be numbers

Array Keywords

// items
.items(false)                         // Array must be empty
.items(true)                          // Any items allowed
.items(s => s.string())               // All items must be strings

// additionalItems (Draft 7 and earlier)
.additionalItems(false)               // No additional items beyond prefixItems
.additionalItems(true)                // Any additional items allowed
.additionalItems(s => s.number())     // Additional items must be numbers

// unevaluatedItems
.unevaluatedItems(false)              // No unevaluated items allowed
.unevaluatedItems(true)               // Any unevaluated items allowed
.unevaluatedItems(s => s.string())    // Unevaluated must be strings

// contains
.contains(true)                       // Array must have at least one item
.contains(false)                      // Always fails (array can't contain anything)
.contains(s => s.number().minimum(10)) // Must contain number >= 10

Composition Keywords

// not
.not(true)                            // Always fails (not true = false)
.not(false)                           // Always passes (not false = true)
.not(s => s.string())                 // Must not be a string

// In anyOf, allOf, oneOf
.anyOf(
  true,                               // Always matches
  s => s.string(),
  { type: "number" }                  // Plain JSON also works
)

Basically in json schema, a boolean is a schema so any keyword that accepts a schema also accepts a boolean.

Composition

allOf - Schema Intersection

All schemas must validate (AND logic):

// Combine multiple schemas
const strictStringSchema = new SchemaBuilder()
  .allOf(
    (s) => s.string().minLength(5),
    (s) => s.string().maxLength(10),
    (s) => s.string().pattern("^[A-Z]"),
  )
  .build();

// Result: Must be 5-10 chars AND start with uppercase
// Valid: "Hello", "World"
// Invalid: "hi" (too short), "hello" (no uppercase start)

Practical Example - Combining Base + Extension:

const baseUserSchema = new SchemaBuilder()
  .object()
  .properties({
    id: (s) => s.integer(),
    name: (s) => s.string(),
  })
  .required(["id", "name"]);

const adminUserSchema = new SchemaBuilder()
  .allOf(
    (s) => s.extend(baseUserSchema), // Include all base properties
    (s) =>
      s
        .object()
        .properties({
          permissions: (s) => s.array().items((s) => s.string()),
          adminLevel: (s) => s.integer().minimum(1),
        })
        .required(["permissions"]),
  )
  .build();

// Result: Admin has id, name, permissions, adminLevel

anyOf - Schema Union

At least one schema must validate (OR logic):

// Accept string OR number
const flexibleIdSchema = new SchemaBuilder()
  .anyOf(
    (s) => s.string().pattern("^[A-Z]{3}\\d{3}$"), // e.g., "ABC123"
    (s) => s.integer().minimum(1000), // e.g., 1234
  )
  .build();

// Valid: "ABC123" or 5000
// Invalid: "abc123" (lowercase), 500 (too small)

Practical Example - Payment Methods:

const paymentMethodSchema = new SchemaBuilder()
  .object()
  .properties({
    type: (s) => s.string(),
  })
  .anyOf(
    // Credit card payment
    (s) =>
      s
        .object()
        .properties({
          type: (s) => s.const("credit_card"),
          cardNumber: (s) => s.string(),
          cvv: (s) => s.string(),
        })
        .required(["type", "cardNumber", "cvv"]),

    // Bank transfer
    (s) =>
      s
        .object()
        .properties({
          type: (s) => s.const("bank_transfer"),
          accountNumber: (s) => s.string(),
          routingNumber: (s) => s.string(),
        })
        .required(["type", "accountNumber", "routingNumber"]),

    // PayPal
    (s) =>
      s
        .object()
        .properties({
          type: (s) => s.const("paypal"),
          email: (s) => s.string().format("email"),
        })
        .required(["type", "email"]),
  )
  .build();

oneOf - Exclusive Schema Union

Exactly one schema must validate (XOR logic):

// Must be EITHER a short string OR a long string, not both
const stringLengthSchema = new SchemaBuilder()
  .oneOf(
    (s) => s.string().maxLength(10), // Short string
    (s) => s.string().minLength(50), // Long string
  )
  .build();

// Valid: "hello" (short), "this is a very long string..." (long)
// Invalid: "medium length string" (matches neither or both)

Practical Example - Discriminated Union:

const shapeSchema = new SchemaBuilder()
  .oneOf(
    // Circle
    (s) =>
      s
        .object()
        .properties({
          type: (s) => s.const("circle"),
          radius: (s) => s.number().minimum(0),
        })
        .required(["type", "radius"]),

    // Rectangle
    (s) =>
      s
        .object()
        .properties({
          type: (s) => s.const("rectangle"),
          width: (s) => s.number().minimum(0),
          height: (s) => s.number().minimum(0),
        })
        .required(["type", "width", "height"]),

    // Triangle
    (s) =>
      s
        .object()
        .properties({
          type: (s) => s.const("triangle"),
          base: (s) => s.number().minimum(0),
          height: (s) => s.number().minimum(0),
        })
        .required(["type", "base", "height"]),
  )
  .build();

// Valid: { type: "circle", radius: 5 }
// Invalid: { type: "circle", radius: 5, width: 10 } (matches multiple)

not - Schema Negation

Schema must NOT validate:

// Not a string
const notStringSchema = new SchemaBuilder().not((s) => s.string()).build();

// Valid: 123, true, {}, []
// Invalid: "hello"

Practical Example - Exclusion Pattern:

// Accept any string EXCEPT email addresses
const nonEmailStringSchema = new SchemaBuilder()
  .string()
  .not((s) => s.string().format("email"))
  .build();

// Valid: "hello", "test123"
// Invalid: "user@example.com"

Complex Composition

const advancedSchema = new SchemaBuilder()
  .object()
  .properties({
    value: (s) => s.string(),
  })
  .allOf(
    // Must have 'value' property
    (s) => s.object().required(["value"]),
  )
  .anyOf(
    // Either has 'type' property
    (s) =>
      s
        .object()
        .properties({
          type: (s) => s.string(),
        })
        .required(["type"]),

    // Or has 'category' property
    (s) =>
      s
        .object()
        .properties({
          category: (s) => s.string(),
        })
        .required(["category"]),
  )
  .not(
    // But NOT both at the same time
    (s) => s.object().required(["type", "category"]),
  )
  .build();

// Valid: { value: "x", type: "a" }
// Valid: { value: "x", category: "b" }
// Invalid: { value: "x", type: "a", category: "b" } (has both)

Conditionals

if/then/else - Basic Conditional Logic

// If type is 'user', then require 'username'
const schema = new SchemaBuilder()
  .object()
  .properties({
    type: (s) => s.string(),
    username: (s) => s.string(),
    apiKey: (s) => s.string(),
  })
  .if((s) =>
    s.object().properties({
      type: (s) => s.const("user"),
    }),
  )
  .then((s) => s.object().required(["username"]))
  .else((s) => s.object().required(["apiKey"]))
  .build();

// If type="user": must have username
// Otherwise: must have apiKey

elseIf - Multiple Conditions

The elseIf extension allows clean chaining without deep nesting:

const accountSchema = new SchemaBuilder()
  .object()
  .properties({
    accountType: (s) => s.string(),
    username: (s) => s.string(),
    email: (s) => s.string(),
    companyName: (s) => s.string(),
    taxId: (s) => s.string(),
  })
  .required(["accountType"])
  .if((s) =>
    s.object().properties({
      accountType: (s) => s.const("personal"),
    }),
  )
  .then((s) => s.object().required(["username", "email"]))
  .elseIf((s) =>
    s.object().properties({
      accountType: (s) => s.const("business"),
    }),
  )
  .then((s) => s.object().required(["companyName", "taxId", "email"]))
  .elseIf((s) =>
    s.object().properties({
      accountType: (s) => s.const("enterprise"),
    }),
  )
  .then((s) => s.object().required(["companyName", "taxId"]))
  .else((s) => s.object().required(["email"]))
  .build();

// Generates {if: schema, then:schema, elseIf[{if: schema, then:schema}, {if: schema, then:schema}], else: schema}

// That structure is perfectly handled by jet-Validator during compilation.

Without elseIf (standard JSON Schema):

// Standard approach - deeply nested
.if(...)
  .then(...)
  .else(s => s
    .if(...)
      .then(...)
      .else(s => s
        .if(...)
          .then(...)
        .else(...)
      )
  )
// Nested hell!

Nested Conditionals

const advancedRulesSchema = new SchemaBuilder()
  .object()
  .properties({
    country: (s) => s.string(),
    state: (s) => s.string(),
    zipCode: (s) => s.string(),
    postalCode: (s) => s.string(),
  })
  .if((s) =>
    s.object().properties({
      country: (s) => s.const("US"),
    }),
  )
  .then(
    (s) =>
      s
        .object()
        .required(["state", "zipCode"])
        .if((s) =>
          s.object().properties({
            state: (s) => s.const("California"),
          }),
        )
        .then((s) =>
          s.object().properties({
            zipCode: (s) => s.string().pattern("^9[0-6]\\d{3}$"),
          }),
        )
        .end(), // Only call .end if you don't need else.
  )
  .elseIf((s) =>
    s.object().properties({
      country: (s) => s.const("UK"),
    }),
  )
  .then((s) => s.object().required(["postalCode"]))
  .end()
  .build();

If you are not ending your conditional with .else() then always call the .end() method

Check Jet-Validator documentation for keywords that support $data

References

$ref - Schema References

Local References

const schema = new SchemaBuilder()
  .$defs({
    address: (s) =>
      s.object().properties({
        street: (s) => s.string(),
        city: (s) => s.string(),
        zipCode: (s) => s.string(),
      }),
  })
  .object()
  .properties({
    billingAddress: (s) => s.$ref("#/$defs/address"),
    shippingAddress: (s) => s.$ref("#/$defs/address"),
  })
  .build();

External References

// Reference external schema by URL
const userSchema = new SchemaBuilder()
  .object()
  .properties({
    profile: (s) => s.$ref("https://example.com/schemas/profile.json"),
  })
  .build();

// Reference with fragment
const schema = new SchemaBuilder()
  .object()
  .properties({
    user: (s) => s.$ref("https://example.com/schemas/common.json#/$defs/user"),
  })
  .build();

RefBuilder - Type-Safe References

The RefBuilder provides a fluent API for constructing JSON Pointer references:

import { RefBuilder } from "@jetio/schema-builder";

// Basic usage
const ref1 = new RefBuilder().$defs("user").properties("address").build();
// Result: "#/$defs/user/properties/address"

// Using in schema
const schema = new SchemaBuilder()
  .object()
  .properties({
    mainUser: (s) => s.$ref((r) => r.$defs("user")),
    altUser: (s) => s.$ref((r) => r.$defs("user").properties("email")),
  })
  .build();

RefBuilder API:

// Definitions
.$defs("schemaName")           // #/$defs/schemaName
.definitions("schemaName")     // #/definitions/schemaName

// Properties
.properties("propName")        // /properties/propName
.patternProperties("pattern")  // /patternProperties/pattern
.additionalProperties()        // /additionalProperties
.propertyNames()              // /propertyNames

// Arrays
.items()                      // /items
.items(0)                     // /items/0
.prefixItems(0)               // /prefixItems/0
.contains()                   // /contains
.additionalItems()            // /additionalItems

// Composition
.allOf(0)                     // /allOf/0
.anyOf(1)                     // /anyOf/1
.oneOf(2)                     // /oneOf/2
.not()                        // /not

// Conditionals
.if()                         // /if
.then()                       // /then
.else()                       // /else
.elseIf(0)                    // /elseIf/0

// Dependencies
.dependentSchemas("prop")     // /dependentSchemas/prop

// Anchors
.anchor("anchorName")         // #anchorName
.dynamicAnchor("name")        // #name

// Base URL
.base("https://example.com/schema") // Change base URL

// Custom segments
.segment("customPath")        // /customPath

// Utilities
.reset()                      // Clear path back to base
.chain()                      // Return self (for external composition)
.extend()                     // Clone for branching

// Building
.build()                      // Returns the complete reference string
.toString()                   // Alias for build()

Complex RefBuilder Example:

const complexSchema = new SchemaBuilder()
  .$defs({
    person: (s) =>
      s.object().properties({
        name: (s) => s.string(),
        contacts: (s) =>
          s.array().items((s) =>
            s.object().properties({
              type: (s) => s.string(),
              value: (s) => s.string(),
            }),
          ),
      }),
  })
  .object()
  .properties({
    // Reference nested property
    primaryContact: (s) =>
      s.$ref((r) => r.$defs("person").properties("contacts").items()),

    // Reference with external base
    externalRef: (s) =>
      s.$ref((r) =>
        r
          .base("https://example.com/schema")
          .$defs("common")
          .properties("metadata"),
      ),
  })
  .build();

RefBuilder with Anchors:

// Using anchors
const schema = new SchemaBuilder()
  .$id("https://example.com/schema")
  .$defs({
    user: (s) =>
      s
        .object()
        .$anchor("userSchema")
        .properties({
          name: (s) => s.string(),
        }),
  })
  .object()
  .properties({
    // Reference by anchor
    admin: (s) => s.$ref((r) => r.anchor("userSchema")),

    // Or by path
    moderator: (s) => s.$ref((r) => r.$defs("user")),
  })
  .build();

RefBuilder Extension:

// Reuse partial refs
const baseRef = new RefBuilder()
  .base("https://api.example.com/schemas")
  .$defs("common");

// Extend for different endpoints
const userRef = baseRef.extend().properties("user").build();
// "https://api.example.com/schemas#/$defs/common/properties/user"

const productRef = baseRef.extend().properties("product").build();
// "https://api.example.com/schemas#/$defs/common/properties/product"

$dynamicRef - Dynamic References

const schema = new SchemaBuilder()
  .$dynamicAnchor("meta")
  .object()
  .properties({
    // Dynamic reference resolves at validation time
    nested: (s) => s.$dynamicRef((r) => r.anchor("meta")),
  })
  .build();

Anchors

Define anchors for internal references:

const schema = new SchemaBuilder()
  .$id("https://example.com/schema")
  .$defs({
    user: (s) =>
      s
        .object()
        .$anchor("simpleUser") // Reference as #simpleUser
        .$dynamicAnchor("recursiveUser") // Dynamic reference
        .properties({
          name: (s) => s.string(),
        }),
  })
  .object()
  .properties({
    user: (s) => s.$ref("#simpleUser"),
  })
  .build();

Definitions

$defs - Schema Definitions

Define reusable schema components:

const schema = new SchemaBuilder()
  .$defs({
    // Define multiple schemas
    emailString: (s) => s.string().format("email"),

    phoneString: (s) => s.string().pattern("^\\+?[1-9]\\d{1,14}$"),

    address: (s) =>
      s
        .object()
        .properties({
          street: (s) => s.string(),
          city: (s) => s.string(),
          zipCode: (s) => s.string(),
        })
        .required(["street", "city"]),

    person: (s) =>
      s.object().properties({
        name: (s) => s.string(),
        email: (s) => s.$ref("#/$defs/emailString"),
        phone: (s) => s.$ref("#/$defs/phoneString"),
        address: (s) => s.$ref("#/$defs/address"),
      }),
  })
  .object()
  .properties({
    customer: (s) => s.$ref("#/$defs/person"),
    billing: (s) => s.$ref("#/$defs/address"),
  })
  .build();

definitions (Legacy)

For JSON Schema Draft 6/7 compatibility:

const draft7Schema = new SchemaBuilder()
  .definitions({
    user: (s) =>
      s.object().properties({
        id: (s) => s.integer(),
        name: (s) => s.string(),
      }),
  })
  .object()
  .properties({
    author: (s) => s.$ref("#/definitions/user"),
  })
  .build();

Custom Error Messages

Add custom error messages to your schemas for better validation feedback:

String Form (All Errors)

Override all validation errors with a single message:

const schema = new SchemaBuilder()
  .string()
  .minLength(5)
  .maxLength(20)
  .pattern("^[a-z]+$")
  .errorMessage("Username must be 5-20 lowercase letters")
  .build();

// ANY validation failure returns: "Username must be 5-20 lowercase letters"

Object Form (Per-Keyword)

Customize error messages for specific validation keywords:

const schema = new SchemaBuilder()
  .string()
  .minLength(5)
  .maxLength(20)
  .pattern("^[a-z]+$")
  .errorMessage({
    type: "Must be text",
    minLength: "Too short - need at least 5 characters",
    maxLength: "Too long - maximum 20 characters",
    pattern: "Only lowercase letters allowed",
  })
  .build();

Property-Level Error Messages

const schema = new SchemaBuilder()
  .object()
  .properties({
    email: (s) => s.string().format("email"),
    age: (s) => s.integer().minimum(18),
  })
  .errorMessage({
    properties: {
      email: "Please enter a valid email address",
      age: "You must be at least 18 years old",
    },
  })
  .build();

Per-Keyword Property Errors

const schema = new SchemaBuilder()
  .object()
  .properties({
    email: (s) => s.string().format("email").minLength(5),
    password: (s) => s.string().minLength(8).pattern("^(?=.*[A-Z])"),
  })
  .errorMessage({
    properties: {
      email: {
        format: "Invalid email format",
        minLength: "Email too short",
      },
      password: {
        minLength: "Password must be at least 8 characters",
        pattern: "Password must contain an uppercase letter",
      },
    },
  })
  .build();

Nested Schema Error Messages

const schema = new SchemaBuilder()
  .object()
  .properties({
    user: (s) =>
      s
        .object()
        .properties({
          name: (s) => s.string().minLength(2),
          email: (s) => s.string().format("email"),
        })
        .errorMessage({
          properties: {
            name: "Name must be at least 2 characters",
            email: "Invalid email",
          },
        }),
  })
  .build();

The `_jetError` Fallback

Use _jetError as a catch-all for keywords without explicit messages:

const schema = new SchemaBuilder()
  .object()
  .properties({
    user: (s) =>
      s.object().properties({
        name: (s) => s.string(),
        email: (s) => s.string(),
      }),
  })
  .errorMessage({
    properties: {
      user: {
        _jetError: "User validation failed", // Fallback
        properties: {
          name: "Invalid name",
          email: "Invalid email",
        },
      },
    },
  })
  .build();

Read the jet-Validator docuemntation to properly understand how error messages works Error Handling.

Advanced Features

Schema Metadata

const schema = new SchemaBuilder()
  .$id("https://example.com/schemas/user.json")
  .$schema("https://json-schema.org/draft/2020-12/schema")
  .title("User Schema")
  .description("Represents a user in the system")
  .object()
  .properties({
    id: (s) => s.integer(),
  })
  .build();

Custom Keywords

Add any custom keyword:

const schema = new SchemaBuilder()
  .object()
  .properties({
    status: (s) => s.string(),
  })
  .option("x-custom-validator", "myValidator")
  .option("x-internal", true)
  .build();

// Result includes: "x-custom-validator": "myValidator"

$data References

Enable runtime data references:

const schema = new SchemaBuilder()
  .object()
  .properties({
    minValue: (s) => s.number(),
    maxValue: (s) => s.number(),
    currentValue: (s) =>
      s
        .number()
        .minimum({ $data: "1/minValue" })
        .maximum({ $data: "1/maxValue" }),
  })
  .build();

// currentValue validated against runtime min/max values

Multiple Schema IDs

const schema = new SchemaBuilder()
  .$id("https://example.com/base")
  .$defs({
    subSchema: (s) =>
      s
        .$id("sub-schema.json") // Relative ID
        .object()
        .properties({
          value: (s) => s.string(),
        }),
  })
  .build();

// Resolves to: https://example.com/sub-schema.json

Schema Reuse & Extension

extend() - Clone & Modify

// Create base schema
const baseUser = new SchemaBuilder()
  .object()
  .properties({
    id: (s) => s.integer(),
    name: (s) => s.string(),
    email: (s) => s.string().format("email"),
  })
  .required(["id", "name"]);

// Extend base schema
const adminUser = new SchemaBuilder()
  .extend(baseUser) // Clone the schema
  .properties({
    role: (s) => s.string().const("admin"),
    permissions: (s) => s.array().items((s) => s.string()),
  })
  .required(["role", "permissions"])
  .build();

// baseUser is unchanged
// adminUser has: id, name, email, role, permissions

Important Notes on Extension

Always extend a schema before building on it. You can replace most keywords by redefining them after extending.

Keywords that MERGE (cannot be replaced):

properties - New properties are added to existing ones
patternProperties - New patterns are added to existing ones
required - New required fields are added to existing array
$defs / definitions - New definitions are added to existing ones
dependentSchemas - New dependent schemas are added to existing ones
dependencies - New dependencies are added to existing ones

Keywords that REPLACE (override existing):

All other keywords (type, oneOf, anyOf, allOf, enum, const, items, etc.)

Modifying merged keywords:

To modify properties from merged keywords, use helper methods:

// Remove properties and update required
const schema = new SchemaBuilder()
  .extend(baseSchema)
  .remove(['fieldName'], ['properties', 'required'])
  .build();

// Make all fields optional
const optionalSchema = new SchemaBuilder()
  .extend(baseSchema)
  .optional()
  .build();

// Override a specific property by redefining it
const updatedSchema = new SchemaBuilder()
  .extend(baseSchema)
  .properties({
    existingField: (s) => s.string() // Redefines the type for this field
  })
  .build();

Example:

const base = new SchemaBuilder()
  .object()
  .properties({
    id: (s) => s.number(),
    name: (s) => s.string()
  })
  .required(['id', 'name'])
  .build();

// This ADDS to properties and required (doesn't replace)
const extended = new SchemaBuilder()
  .extend(base)
  .properties({ email: (s) => s.string() })
  .required(['email'])
  .build();
// Result: properties = { id, name, email }, required = ['id', 'name', 'email']

// To remove a property:
const withoutName = new SchemaBuilder()
  .extend(base)
  .remove(['name'], ['properties'])
  .build();
// Result: properties = { id }, required = ['id']

// To make all optional:
const allOptional = new SchemaBuilder()
  .extend(base)
  .optional()
  .build();
// Result: properties = { id, name }, required = []

See available targets for remove in objects above.

Loading External Schemas

From URL

const schema = await new SchemaBuilder()
  .url("https://example.com/schemas/user.json")
  .extend()
  .properties({
    extraField: (s) => s.string(),
  })
  .build();

From File

const schema = await new SchemaBuilder()
  .file("./schemas/base.json")
  .extend()
  .properties({
    additionalProp: (s) => s.string(),
  })
  .build();

From JSON

const existingSchema = {
  type: "object",
  properties: {
    name: { type: "string" },
  },
};

const extended = new SchemaBuilder()
  .json(existingSchema)
  .extend()
  .properties({
    age: (s) => s.number(),
  })
  .required(["name", "age"])
  .build();

Note: When loading an external schema with either file, url or json, ensure its done at the beginning of the schema builder as it replaces the entire schema object of that schema builder. Ensure its done first before building on the schema. This rule also applies to extend.

Composition with extend()

const timestampMixin = new SchemaBuilder().object().properties({
  createdAt: (s) => s.string().format("date-time"),
  updatedAt: (s) => s.string().format("date-time"),
});

const auditMixin = new SchemaBuilder().object().properties({
  createdBy: (s) => s.string(),
  modifiedBy: (s) => s.string(),
});

const fullSchema = new SchemaBuilder()
  .object()
  .properties({
    id: (s) => s.integer(),
    data: (s) => s.string(),
  })
  .allOf(
    (s) => s.extend(timestampMixin),
    (s) => s.extend(auditMixin),
  )
  .build();

// Result has: id, data, createdAt, updatedAt, createdBy, modifiedBy

Mixed Approaches

You can freely mix JSON objects with builder syntax:

JSON in Properties

const schema = new SchemaBuilder()
  .object()
  .properties({
    // Plain JSON
    simpleField: { type: "string", minLength: 5 },

    // Builder syntax
    complexField: (s) =>
      s.object().properties({
        nested: (s) => s.number(),
      }),

    // Mix in same schema
    mixed: {
      type: "object",
      properties: {
        plain: { type: "string" },
      },
    },
  })
  .build();

JSON in Definitions

const schema = new SchemaBuilder()
  .$defs({
    // Plain JSON
    simpleType: { type: "string", format: "email" },

    // Builder
    complexType: (s) =>
      s.object().properties({
        value: (s) => s.number(),
      }),

    // Nested mix
    mixedType: {
      type: "object",
      properties: {
        items: {
          type: "array",
          items: { type: "string" },
        },
      },
    },
  })
  .build();

JSON in Composition

const schema = new SchemaBuilder()
  .anyOf(
    // Plain JSON
    { type: "string", minLength: 5 },

    // Builder
    (s) => s.number().minimum(0),

    // Boolean
    true,

    // Mixed
    {
      type: "object",
      properties: {
        special: { type: "boolean" },
      },
    },
  )
  .build();

When to Use Each

Use Builder When:

Building schemas programmatically
Need autocomplete/type safety
Complex nested structures
Reusing schema components
Dynamic schema generation

Use JSON When:

Copying from existing JSON Schema
Performance-critical
Interfacing with external systems

Mix When:

Migrating existing schemas
Some parts are static, others dynamic
Balancing readability and flexibility

Complete Examples

E-Commerce Product Schema

const productSchema = new SchemaBuilder()
  .$id("https://example.com/schemas/product.json")
  .$defs({
    price: (s) =>
      s
        .object()
        .properties({
          amount: (s) => s.number().minimum(0),
          currency: (s) => s.string().pattern("^[A-Z]{3}$"),
        })
        .required(["amount", "currency"]),

    dimensions: (s) =>
      s
        .object()
        .properties({
          length: (s) => s.number().minimum(0),
          width: (s) => s.number().minimum(0),
          height: (s) => s.number().minimum(0),
          unit: (s) => s.string().enum(["cm", "in", "m"]),
        })
        .required(["length", "width", "height", "unit"]),
  })
  .object()
  .properties({
    id: (s) => s.string().pattern("^PROD-\\d{6}$"),
    name: (s) => s.string().minLength(3).maxLength(100),
    description: (s) => s.string().maxLength(1000),
    category: (s) =>
      s.string().enum(["electronics", "clothing", "food", "books"]),
    price: (s) => s.$ref("#/$defs/price"),
    stock: (s) => s.integer().minimum(0),
    dimensions: (s) => s.$ref("#/$defs/dimensions"),
    tags: (s) =>
      s
        .array()
        .items((s) => s.string())
        .minItems(1)
        .maxItems(10)
        .uniqueItems(true),
    availability: (s) =>
      s.string().enum(["in_stock", "out_of_stock", "preorder"]),
  })
  .required(["id", "name", "category", "price", "stock", "availability"])
  .if((s) =>
    s.object().properties({
      category: (s) => s.const("electronics"),
    }),
  )
  .then((s) =>
    s
      .object()
      .properties({
        warranty: (s) =>
          s
            .object()
            .properties({
              months: (s) => s.integer().minimum(0),
              type: (s) => s.string().enum(["limited", "full"]),
            })
            .required(["months", "type"]),
      })
      .required(["warranty"]),
  )
  .elseIf((s) =>
    s.object().properties({
      category: (s) => s.const("clothing"),
    }),
  )
  .then((s) =>
    s
      .object()
      .properties({
        sizes: (s) =>
          s
            .array()
            .items((s) => s.string().enum(["XS", "S", "M", "L", "XL", "XXL"]))
            .minItems(1),
        colors: (s) =>
          s
            .array()
            .items((s) => s.string())
            .minItems(1),
      })
      .required(["sizes", "colors"]),
  )
  .build();

API Configuration Schema

const apiConfigSchema = new SchemaBuilder()
  .$defs({
    endpoint: (s) =>
      s
        .object()
        .properties({
          url: (s) => s.string().format("uri"),
          method: (s) =>
            s.string().enum(["GET", "POST", "PUT", "DELETE", "PATCH"]),
          timeout: (s) => s.integer().minimum(100).maximum(30000),
          retries: (s) => s.integer().minimum(0).maximum(5),
        })
        .required(["url", "method"]),

    auth: (s) =>
      s
        .object()
        .properties({
          type: (s) => s.string(),
        })
        .if((s) => s.object().properties({ type: (s) => s.const("bearer") }))
        .then((s) =>
          s
            .object()
            .properties({
              token: (s) => s.string(),
            })
            .required(["token"]),
        )
        .elseIf((s) => s.object().properties({ type: (s) => s.const("basic") }))
        .then((s) =>
          s
            .object()
            .properties({
              username: (s) => s.string(),
              password: (s) => s.string(),
            })
            .required(["username", "password"]),
        )
        .elseIf((s) =>
          s.object().properties({ type: (s) => s.const("apikey") }),
        )
        .then((s) =>
          s
            .object()
            .properties({
              key: (s) => s.string(),
              header: (s) => s.string().default("X-API-Key"),
            })
            .required(["key"]),
        )
        .end()
        .required(["type"]),
  })
  .object()
  .properties({
    service: (s) => s.string().minLength(1),
    baseUrl: (s) => s.string().format("uri"),
    auth: (s) => s.$ref("#/$defs/auth"),
    endpoints: (s) =>
      s
        .object()
        .patternProperties({
          "^[a-zA-Z][a-zA-Z0-9_]*$": (s) => s.$ref("#/$defs/endpoint"),
        })
        .minProperties(1),
    logging: (s) =>
      s.object().properties({
        enabled: (s) => s.boolean().default(true),
        level: (s) => s.string().enum(["debug", "info", "warn", "error"]),
      }),
  })
  .required(["service", "baseUrl", "auth", "endpoints"])
  .build();

Form Validation Schema

const formSchema = new SchemaBuilder()
  .object()
  .properties({
    username: (s) =>
      s
        .string()
        .minLength(3)
        .maxLength(20)
        .pattern("^[a-zA-Z0-9_]+$")
        .title("Username")
        .description("Alphanumeric characters and underscores only")
        .errorMessage({
          minLength: "Username must be at least 3 characters",
          maxLength: "Username cannot exceed 20 characters",
          pattern:
            "Username can only contain letters, numbers, and underscores",
        }),

    email: (s) =>
      s
        .string()
        .format("email")
        .title("Email Address")
        .errorMessage("Please enter a valid email address"),

    password: (s) =>
      s
        .string()
        .minLength(8)
        .pattern("^(?=.*[a-z])(?=.*[A-Z])(?=.*\\d)")
        .title("Password")
        .description("Must contain uppercase, lowercase, and number")
        .errorMessage({
          minLength: "Password must be at least 8 characters",
          pattern: "Password must contain uppercase, lowercase, and a number",
        }),

    confirmPassword: (s) =>
      s
        .string()
        .const({ $data: "1/password" })
        .title("Confirm Password")
        .errorMessage("Passwords do not match"),

    age: (s) =>
      s.integer().minimum(13).maximum(120).title("Age").errorMessage({
        minimum: "You must be at least 13 years old",
        maximum: "Please enter a valid age",
      }),

    country: (s) => s.string().enum(["US", "UK", "CA", "AU"]).title("Country"),

    agreedToTerms: (s) =>
      s
        .boolean()
        .const(true)
        .title("I agree to the terms and conditions")
        .errorMessage("You must agree to the terms and conditions"),
  })
  .required([
    "username",
    "email",
    "password",
    "confirmPassword",
    "age",
    "agreedToTerms",
  ])
  .if((s) =>
    s.object().properties({
      country: (s) => s.const("US"),
    }),
  )
  .then((s) =>
    s
      .object()
      .properties({
        state: (s) =>
          s
            .string()
            .pattern("^[A-Z]{2}$")
            .title("State")
            .errorMessage("Please enter a valid 2-letter state code"),
      })
      .required(["state"]),
  )
  .build();

Best Practices

1. Use Callbacks for Nested Schemas

// ✅ Good - clean and readable
.properties({
  user: s => s.object().properties({
    name: s => s.string()
  })
})

// ❌ Avoid - harder to read
.properties({
  user: new SchemaBuilder().object().properties(...)
})

2. Extract Complex Schemas

// ✅ Good - reusable
const addressSchema = (s: SchemaBuilder) =>
  s.object().properties({
    street: (s) => s.string(),
    city: (s) => s.string(),
  });

const schema = new SchemaBuilder()
  .object()
  .properties({
    billing: addressSchema,
    shipping: addressSchema,
  })
  .build();

3. Use $defs for Reusable Components

// ✅ Good - DRY principle
.$defs({
  email: s => s.string().format("email")
})
.properties({
  primary: s => s.$ref("#/$defs/email"),
  secondary: s => s.$ref("#/$defs/email")
})

4. Leverage RefBuilder for Complex References

// ✅ Good - type-safe
.$ref(r => r.$defs("user").properties("address"))

// ❌ Avoid - error-prone
.$ref("#/$defs/user/properties/address")

5. Use extend() for Schema Composition

// ✅ Good - composable
const base = new SchemaBuilder().object().properties({...});
const extended = new SchemaBuilder().extend(base).properties({...}).build();

6. Add Error Messages for User-Facing Validation

// ✅ Good - helpful error messages
.string()
.minLength(8)
.errorMessage("Password must be at least 8 characters")

// ❌ Avoid - cryptic default errors for end users
.string()
.minLength(8)

Tips & Tricks

Chaining Multiple Types

// Creates type: ["string", "number", "null"]
new SchemaBuilder()
  .null()
  .number()
  .string()
  .minLength(5) // Only applied when value is string
  .build();

Building Incrementally

let schema = new SchemaBuilder().object();

if (requireAuth) {
  schema = schema.properties({
    token: (s) => s.string(),
  });
}

if (requireAdmin) {
  schema = schema.properties({
    adminKey: (s) => s.string(),
  });
}

const final = schema.build();

Common Patterns

Discriminated Unions

const eventSchema = new SchemaBuilder()
  .object()
  .properties({
    type: (s) => s.string(),
  })
  .required(["type"])
  .oneOf(
    (s) =>
      s.object().properties({
        type: (s) => s.const("click"),
        x: (s) => s.number(),
        y: (s) => s.number(),
      }),
    (s) =>
      s.object().properties({
        type: (s) => s.const("scroll"),
        scrollY: (s) => s.number(),
      }),
  )
  .build();

Pagination

const paginatedSchema = (itemSchema: (s: SchemaBuilder) => SchemaBuilder) =>
  new SchemaBuilder()
    .object()
    .properties({
      items: (s) => s.array().items(itemSchema),
      total: (s) => s.integer().minimum(0),
      page: (s) => s.integer().minimum(1),
      pageSize: (s) => s.integer().minimum(1).maximum(100),
    })
    .required(["items", "total", "page", "pageSize"])
    .build();

Polymorphic IDs

const idSchema = new SchemaBuilder()
  .anyOf(
    (s) => s.string().pattern("^[0-9a-f]{24}$"), // MongoDB ObjectId
    (s) => s.string().format("uuid"), // UUID
    (s) => s.integer().minimum(1), // Integer ID
  )
  .build();

API Reference

SchemaBuilder Methods

Type Methods

Method	Returns	Description
`.string()`	`StringSchemaBuilder`	Set type to string
`.number()`	`NumberSchemaBuilder`	Set type to number
`.integer()`	`NumberSchemaBuilder`	Set type to integer
`.boolean()`	`BooleanSchema`	Set type to boolean
`.null()`	`NullSchema`	Set type to null
`.array()`	`ArraySchemaBuilder`	Set type to array
`.object()`	`ObjectSchemaBuilder`	Set type to object

Metadata Methods

Method	Parameters	Description
`.$id(id)`	`string`	Set $id (Draft 6+)
`.$schema(uri)`	`string`	Set $schema
`.$anchor(name)`	`string`	Set $anchor
`.$dynamicAnchor(name)`	`string`	Set $dynamicAnchor
`.title(title)`	`string`	Set title
`.description(desc)`	`string`	Set description
`.default(value)`	`any`	Set default value
`.examples(...values)`	`any[]`	Set examples
`.option(key, value)`	`string, any`	Set any custom keyword

Value Constraint Methods

Method	Parameters	Description
`.enum(values)`	`any[]` or `$data`	Set allowed values
`.const(value)`	`any`	Set single allowed value

Composition Methods

Method	Parameters	Description
`.not(schema)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	Negate a schema
`.anyOf(...schemas)`	`BuilderSchema[]` or `((b: SchemaBuilder) => SchemaBuilder)[]`	Match at least one schema
`.allOf(...schemas)`	`BuilderSchema[]` or `((b: SchemaBuilder) => SchemaBuilder)[]`	Match all schemas
`.oneOf(...schemas)`	`BuilderSchema[]` or `((b: SchemaBuilder) => SchemaBuilder)[]`	Match exactly one schema

Note: BuilderSchema = SchemaDefinition | SchemaBuilder<any> | boolean

Conditional Methods

Method	Parameters	Returns	Description
`.if(condition)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	`ConditionBuilder`	Start conditional

Reference Methods

Method	Parameters	Description
`.$ref(ref)`	`string` or `RefBuilder` or `(r: RefBuilder) => RefBuilder`	Set $ref
`.$dynamicRef(ref)`	`string` or `RefBuilder` or `(r: RefBuilder) => RefBuilder`	Set $dynamicRef
`.$defs(defs)`	`Record<string, BuilderSchema>` or `Record<string, (b: SchemaBuilder) => SchemaBuilder>`	Set $defs (Draft 2019-09+)
`.definitions(defs)`	`Record<string, BuilderSchema>` or `Record<string, (b: SchemaBuilder) => SchemaBuilder>`	Set definitions (Draft 04-07)

Error Message Methods

Method	Parameters	Description
`.errorMessage(msg)`	`string` or `Record<string, string \| Record<string, string>>`	Set custom error message

Utility Methods

Method	Parameters	Returns	Description
`.build()`	-	`S`	Return the final schema object
`.extend(schema)`	`BuilderSchema`	`SchemaBuilder`	Extend from existing schema
`.json(schema)`	`SchemaDefinition` or `string`	`SchemaBuilder`	Load from JSON object/string
`.url(url)`	`string`	`Promise<SchemaBuilder>`	Load from URL (async)
`.file(path)`	`string`	`Promise<SchemaBuilder>`	Load from file (async)

StringSchemaBuilder Methods

Inherits all SchemaBuilder methods, plus:

Method	Parameters	Returns	Description
`.minLength(len)`	`number` or `$data`	`SchemaBuilder`	Minimum string length
`.maxLength(len)`	`number` or `$data`	`SchemaBuilder`	Maximum string length
`.pattern(regex)`	`string` or `RegExp` or `$data`	`SchemaBuilder`	Regex pattern
`.format(name)`	`string` or `$data`	`SchemaBuilder`	Format validation (email, uri, date-time, etc.)

NumberSchemaBuilder Methods

Inherits all SchemaBuilder methods, plus:

Method	Parameters	Returns	Description
`.minimum(n)`	`number` or `$data`	`SchemaBuilder`	Minimum value (inclusive)
`.maximum(n)`	`number` or `$data`	`SchemaBuilder`	Maximum value (inclusive)
`.exclusiveMinimum(n)`	`number` or `$data`	`SchemaBuilder`	Minimum value (exclusive)
`.exclusiveMaximum(n)`	`number` or `$data`	`SchemaBuilder`	Maximum value (exclusive)
`.multipleOf(n)`	`number` or `$data`	`SchemaBuilder`	Must be multiple of n
`.positive()`	-	`SchemaBuilder`	Shorthand for minimum(0)
`.negative()`	-	`SchemaBuilder`	Shorthand for maximum(0)
`.range(min, max)`	`number` or `$data`, `number` or `$data`	`SchemaBuilder`	Shorthand for minimum + maximum

ArraySchemaBuilder Methods

Inherits all SchemaBuilder methods, plus:

Method	Parameters	Returns	Description
`.items(...schemas)`	`BuilderSchema[]` or `((b: SchemaBuilder) => SchemaBuilder)[]`	`SchemaBuilder`	Schema for all items (or tuple for Draft 07)
`.prefixItems(...schemas)`	`BuilderSchema[]` or `((b: SchemaBuilder) => SchemaBuilder)[]`	`SchemaBuilder`	Tuple validation (Draft 2020-12)
`.additionalItems(schema)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	`SchemaBuilder`	Schema for items beyond tuple (Draft 07)
`.unevaluatedItems(schema)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	`SchemaBuilder`	Schema for unevaluated items (Draft 2019-09+)
`.contains(schema)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	`SchemaBuilder`	Array must contain matching item
`.minContains(n)`	`number` or `$data`	`SchemaBuilder`	Minimum matching items
`.maxContains(n)`	`number` or `$data`	`SchemaBuilder`	Maximum matching items
`.minItems(n)`	`number` or `$data`	`SchemaBuilder`	Minimum array length
`.maxItems(n)`	`number` or `$data`	`SchemaBuilder`	Maximum array length
`.uniqueItems(value)`	`boolean` or `$data`	`SchemaBuilder`	Require unique items

ObjectSchemaBuilder Methods

Inherits all SchemaBuilder methods, plus:

Method	Parameters	Returns	Description
`.properties(props)`	`Record<string, BuilderSchema>` or `Record<string, (b: SchemaBuilder) => SchemaBuilder>`	`SchemaBuilder`	Define property schemas
`.patternProperties(patterns)`	`Record<string, BuilderSchema>` or `Record<string, (b: SchemaBuilder) => SchemaBuilder>`	`SchemaBuilder`	Define pattern-matched property schemas
`.additionalProperties(schema)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	`SchemaBuilder`	Schema for additional properties
`.unevaluatedProperties(schema)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	`SchemaBuilder`	Schema for unevaluated properties (Draft 2019-09+)
`.propertyNames(schema)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	`SchemaBuilder`	Schema for property names
`.required(fields)`	`string[]` or `$data`	`SchemaBuilder`	Required property names
`.minProperties(n)`	`number` or `$data`	`SchemaBuilder`	Minimum property count
`.maxProperties(n)`	`number` or `$data`	`SchemaBuilder`	Maximum property count
`.dependentRequired(deps)`	`Record<string, string[]>`	`SchemaBuilder`	Conditional required fields
`.dependentSchemas(deps)`	`Record<string, BuilderSchema>` or `Record<string, (b: SchemaBuilder) => SchemaBuilder>`	`SchemaBuilder`	Conditional schemas (Draft 2019-09+)
`.dependencies(deps)`	`Record<string, BuilderSchema \| string[]>` or `Record<string, (b: SchemaBuilder) => SchemaBuilder \| string[]>`	`SchemaBuilder`	Combined dependencies (Draft 07)
`.remove(properties, targets?)`	`string[]`, `("properties" \| "required" \| "patternProperties" \| "dependencies" \| "dependentRequired")[]?`	`SchemaBuilder`	Remove properties from schema
`.optional()`	-	`SchemaBuilder`	Remove all required constraints

RefBuilder Methods

Method	Parameters	Returns	Description
`.base(url)`	`string`	`RefBuilder`	Set base URL
`.$defs(name)`	`string`	`RefBuilder`	Add /$defs/name
`.definitions(name)`	`string`	`RefBuilder`	Add /definitions/name
`.properties(name)`	`string`	`RefBuilder`	Add /properties/name
`.patternProperties(pattern)`	`string`	`RefBuilder`	Add /patternProperties/pattern
`.additionalProperties()`	-	`RefBuilder`	Add /additionalProperties
`.unevaluatedProperties()`	-	`RefBuilder`	Add /unevaluatedProperties
`.propertyNames()`	-	`RefBuilder`	Add /propertyNames
`.dependentSchemas(prop)`	`string`	`RefBuilder`	Add /dependentSchemas/prop
`.dependencies(prop)`	`string`	`RefBuilder`	Add /dependencies/prop
`.items(index?)`	`number?`	`RefBuilder`	Add /items or /items/index
`.prefixItems(index)`	`number`	`RefBuilder`	Add /prefixItems/index
`.additionalItems()`	-	`RefBuilder`	Add /additionalItems
`.unevaluatedItems()`	-	`RefBuilder`	Add /unevaluatedItems
`.contains()`	-	`RefBuilder`	Add /contains
`.allOf(index)`	`number`	`RefBuilder`	Add /allOf/index
`.anyOf(index)`	`number`	`RefBuilder`	Add /anyOf/index
`.oneOf(index)`	`number`	`RefBuilder`	Add /oneOf/index
`.not()`	-	`RefBuilder`	Add /not
`.if()`	-	`RefBuilder`	Add /if
`.then()`	-	`RefBuilder`	Add /then
`.else()`	-	`RefBuilder`	Add /else
`.elseIf(index)`	`number`	`RefBuilder`	Add /elseIf/index
`.anchor(name)`	`string`	`RefBuilder`	Set anchor reference
`.dynamicAnchor(name)`	`string`	`RefBuilder`	Set dynamic anchor reference
`.segment(path)`	`string`	`RefBuilder`	Add custom path segment
`.reset()`	-	`RefBuilder`	Clear path back to base
`.extend()`	-	`RefBuilder`	Clone RefBuilder
`.chain()`	-	`RefBuilder`	Return self (for external composition)
`.build()`	-	`string`	Return the reference string
`.toString()`	-	`string`	Alias for build()

ConditionBuilder Methods

Method	Parameters	Returns	Description
`.then(schema)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	`ConditionBuilder`	Schema if condition passes
`.elseIf(condition)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	`ConditionBuilder`	Add another condition
`.else(schema)`	`BuilderSchema` or `(b: SchemaBuilder) => SchemaBuilder`	Parent builder	Schema if all conditions fail
`.end()`	-	Parent builder	End conditional without else

Type Definitions

// Core types
type BuilderSchema = SchemaDefinition | SchemaBuilder<any> | boolean;

// $data reference for dynamic values
type $data = { $data: string };

// Schema callback pattern
type SchemaCallback = (builder: SchemaBuilder) => SchemaBuilder;

Common Patterns

Inline Schema Definition

.properties({
  name: (s) => s.string(),
  age: (s) => s.number()
})

Reusable Schema

const nameSchema = new SchemaBuilder().string().build();

.properties({
  name: nameSchema
})

Boolean Schemas

.properties({
  allowAny: true,      // Accepts any value
  rejectAll: false     // Rejects all values
})

Composition Patterns

// Using callbacks
.allOf(
  (s) => s.object().properties({ id: (s) => s.number() }),
  (s) => s.object().properties({ name: (s) => s.string() })
)

// Using existing schemas
const baseSchema = new SchemaBuilder().object().build();
.allOf(baseSchema, otherSchema)

// Mixed
.oneOf(
  baseSchema,
  (s) => s.object().properties({ type: (s) => s.const('custom') })
)

@jetio/schema-builder

Package Exports

Readme

📐 Schema Builder Guide

📦 Installation

Type Inference

Why Type Inference Matters

Advanced Type Inference

🚀 Try It Live

Using with @jetio/validator

🎯 Quick Start

📚 Table of Contents

Draft Support

Basic Types

Simple Type Declarations

Multiple Types

Universal Keywords

TypeScript Type Locking

String Schemas

Length Constraints

Pattern Matching

Format Validation

Number Schemas

Numeric Constraints

Multiple Of

Convenience Methods

Enums & Constants

Array Schemas

Basic Arrays

Tuple Validation (Prefix Items)

Contains & Count Constraints

Complex Array Items

Unevaluated Items (Draft 2019-09+)

Additional Items (Draft 07 and earlier)

Object Schemas

Properties And Required

Pattern Properties

Additional Properties

Unevaluated Properties (Draft 2019-09+)

Property Name Constraints

Property Count

Dependent Schemas

Dependent Required

Dependencies

Nested Objects

Removing Properties

Making Fields Optional

Boolean Schema Values

Object Keywords

Array Keywords

Composition Keywords

Composition

allOf - Schema Intersection

anyOf - Schema Union

oneOf - Exclusive Schema Union

not - Schema Negation

Complex Composition

Conditionals

if/then/else - Basic Conditional Logic

elseIf - Multiple Conditions

Nested Conditionals

References

$ref - Schema References

Local References

External References

RefBuilder - Type-Safe References

$dynamicRef - Dynamic References

Anchors

Definitions

$defs - Schema Definitions

definitions (Legacy)

Custom Error Messages

String Form (All Errors)

Object Form (Per-Keyword)

Property-Level Error Messages

Per-Keyword Property Errors

Nested Schema Error Messages

The _jetError Fallback

Advanced Features

Schema Metadata

The `_jetError` Fallback