Package Exports

typist-json

This package does not declare an exports field, so the exports above have been automatically detected and optimized by JSPM instead. If any package subpath is missing, it is recommended to post an issue to the original package (typist-json) to support the "exports" field. If that is not possible, create a JSPM override to customize the exports field for this package.

Readme

typist-json

A simple runtime JSON type checker.

Features

Simple. No JSON Schema, No validation rules
Type-safe. Written in TypeScript
Intuitive. Familiar syntax like TypeScript interface

Install

npm install typist-json

Example

import { j } from 'typist-json'

const UserJsonType = j.object({
  name: j.string,
  age: j.number,
  'nickname?': j.string, // optional property
})

const userJson = await fetch("/api/user")
    .then(res => res.json)

if (UserJsonType.check(userJson)) {
  // now, the userJson is narrowed to:
  // {
  //   name: string
  //   age: number
  //   nickname?: string | undefined
  // }
}

References

Checker<T>

A base interface that all type checkers implement.

check(value: unknown): value is T

Check whether the value is of type T or not.

If check returns true then the value is of type T and the value is narrowed to T.

j

A container that contains all built-in type checkers.

All built-in type checkers are followings:

j.string

Checks if the value is string type or not.

j.string.check("foo") // true, narrowed to `string`

j.number

Checks if the value is number type or not.

j.number.check(42.195) // true, narrowed to `number`

j.boolean

Checks if the value is boolean type or not.

j.boolean.check(true) // true, narrowed to `boolean`

j.literal(str: string)

Checks if the value equals str or not.

const json: any = "foo"
j.literal("foo").check(json) // true, narrowed to `"foo"`
j.literal("foo").check("bar") // false

j.unknown

Doesn't check the value type. This won't narrow types.

j.unknown.check("foo") // true
j.unknown.check({a: 42}) // true

j.nil

Checks if the value is null or not.

j.nil.check(null) // true, narrowed to `null`

j.nullable(checker: Checker)

Checks if the value is null or matches the checker.

j.nullable(string).check(null) // true, narrowed to `string | null`
j.nullable(string).check("foo") // true, narrowed to `string | null`

j.any(checkers: Checker[])

Checks if the value matches any of the checkers.

const checker = j.any([string, number])
checker.check("foo") // true, narrowed to `string | number`
checker.check(42) // true, narrowed to `string | number`

j.array(checker: Checker)

Checks if the value is an array of elements that match the checker.

j.array(string).check(["foo", "bar"]) // true, narrowed to `string[]`

j.object(properties: {[key: string]: Checker})

Checks if the value is an object that consists of properties and each property matches correspond checkers.

A property name ends with ? is considered optional.

const checker = j.object({
  name: j.string,
  age: j.number,
  'nickname?': j.string,
})

// true, narrowed to { name: string, age: number, nickname?: string | undefined }
checker.ckeck({
  name: "John",
  age: 42,
  nickname: "Johnny",
})

// true, because `nickname` is optional.
// narrowed to { name: string, age: number, nickname?: string | undefined }
checker.ckeck({
  name: "Emma",
  age: 20,
})

// false, because `nickname` is optional, not nullable.
// optional properties are distinguished from nullable properties.
checker.check({
  name: "Bob",
  age: 18,
  nickname: null, // invalid
})

// similarly, nullable properties cannot be omitted.
j.object({ foo: j.nullable(j.string) }).check({}) // false, because property named `foo` is required

If you want to use property name that ends with ? as non-optional property, you can escape ? as ??.

const checker = j.object({
  "are_you_sure??": j.boolean,
})

// true, narrowed to { "are_you_sure?": boolean }
checker.check({
  "are_you_sure?": true,
})

More details about escaping

As mentioned above, you need to escape all trailing ? as ??.

So if you want optional property with a name "foo???", you should use "foo???????" as property name for j.object like:

const checker = j.object({
  "foo???????": j.boolean,
})

// true, narrowed to { "foo???"?: boolean | undefined }
checker.check({
  "foo???": true,
})

JsonTypeOf<Checker>

A type alias that represents the type of JSON corresponding to the Checker.

const checker = j.object({
  model: j.string,
  os: j.any([
    j.literal("ios"),
    j.literal("android"),
    j.literal("blackberry")
  ]),
  serial_number: j.string,
})

type SmartPhoneJson = JsonTypeOf<typeof checker>
// SmartPhoneJson will be:
// {
//   model: string
//   os: "ios" | "android" | "blackberry"
//   serial_number: string
// }

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