@formspec/constraints

Define and enforce constraints on which FormSpec DSL features are allowed in your project.

Overview

The constraints package lets you restrict which parts of the FormSpec DSL can be used. This is useful for:

• Standardization: Enforce consistent form patterns across a team
• Compatibility: Restrict to features your renderer supports
• Simplicity: Keep forms simple by disallowing complex nesting or conditionals
• Linting: Catch constraint violations at development time via ESLint

Installation

npm install @formspec/constraints
# or
pnpm add @formspec/constraints

Configuration

Create a .formspec.yml file in your project root:

constraints:
  fieldTypes:
    text: off # Allow text fields
    number: off # Allow number fields
    boolean: off # Allow boolean fields
    staticEnum: off # Allow static enums
    dynamicEnum: warn # Warn on dynamic enums
    dynamicSchema: error # Disallow dynamic schemas
    array: off # Allow arrays
    object: off # Allow objects

  layout:
    group: off # Allow groups
    conditionals: off # Allow when() conditionals
    maxNestingDepth: 3 # Max nesting depth (0 = flat only)

  fieldOptions:
    label: off
    placeholder: off
    required: off
    minValue: off
    maxValue: off
    minItems: off
    maxItems: off

Severity Levels

Each constraint can be set to:

Constraint Categories

Field Types (fieldTypes)

Control which DSL field builders are allowed:

Layout (layout)

Control structure and nesting:

Field Options (fieldOptions)

Control which field configuration options are allowed:

UI Schema (uiSchema)

Control JSON Forms-specific features:

constraints:
  uiSchema:
    layouts:
      VerticalLayout: off
      HorizontalLayout: off
      Group: off
      Categorization: error # Disallow tabbed interfaces
      Category: error
    rules:
      enabled: off
      effects:
        SHOW: off
        HIDE: off
        ENABLE: warn
        DISABLE: warn

Programmatic Usage

Loading Configuration

import { loadConfig, mergeWithDefaults } from "@formspec/constraints";

// Load from .formspec.yml (searches up directory tree)
const config = await loadConfig();

// Or load from specific path
const config = await loadConfig("/path/to/.formspec.yml");

// Merge with defaults to get fully resolved config
const resolved = mergeWithDefaults(config.constraints);

Validating Forms

import { validateFormSpec } from "@formspec/constraints";
import { formspec, field, when, is } from "@formspec/dsl";

const form = formspec(
  field.text("name"),
  field.dynamicEnum("country", "fetch_countries"),
  when(is("country", "US"), field.text("state"))
);

const result = validateFormSpec(form, resolved);

if (!result.valid) {
  for (const issue of result.issues) {
    console.log(`${issue.severity}: ${issue.message}`);
  }
}

Validation Result

interface ValidationResult {
  valid: boolean; // true if no errors (warnings OK)
  issues: ValidationIssue[];
}

interface ValidationIssue {
  code: string; // e.g., "FIELD_TYPE_NOT_ALLOWED"
  message: string; // Human-readable description
  severity: "error" | "warning";
  category: "fieldTypes" | "layout" | "uiSchema" | "fieldOptions" | "controlOptions";
  path?: string; // JSON pointer to issue location
  fieldName?: string; // Affected field name
  fieldType?: string; // Affected field type
}

ESLint Integration

Use @formspec/eslint-plugin to catch constraint violations at development time:

// eslint.config.js
import formspec from "@formspec/eslint-plugin";

export default [
  {
    plugins: { formspec },
    rules: {
      // Enforce allowed field types from .formspec.yml
      "formspec/constraints-allowed-field-types": "error",
      // Enforce allowed layouts from .formspec.yml
      "formspec/constraints-allowed-layouts": "error",
    },
  },
];

The ESLint rules automatically load constraints from your .formspec.yml file.

Example Configurations

Simple Forms Only

Restrict to flat forms with basic field types:

constraints:
  fieldTypes:
    array: error
    object: error
    dynamicEnum: error
    dynamicSchema: error
  layout:
    conditionals: error
    maxNestingDepth: 0

JSON Forms Compatible

Restrict to features supported by standard JSON Forms renderers:

constraints:
  fieldTypes:
    dynamicSchema: error # Not supported by JSON Forms
  uiSchema:
    layouts:
      Categorization: warn # May not be supported by all renderers

Warn on Advanced Features

Allow all features but warn on complex ones:

constraints:
  fieldTypes:
    dynamicEnum: warn
    dynamicSchema: warn
    array: warn
    object: warn
  layout:
    conditionals: warn
    maxNestingDepth: 2

JSON Schema

A JSON Schema for .formspec.yml is available for editor autocompletion:

# .formspec.yml
# yaml-language-server: $schema=node_modules/@formspec/constraints/formspec.schema.json

constraints:
  fieldTypes:
    text: off
    # ... autocomplete available