json-schema-to-ts

Stop typing twice 🙅‍♂️

A lot of projects use JSON schemas for runtime data validation along with TypeScript for static type checking.

Their code may look like this:

const dogSchema = {
  type: "object",
  properties: {
    name: { type: "string" },
    age: { type: "integer" },
    hobbies: { type: "array", items: { type: "string" } },
    favoriteFood: { enum: ["pizza", "taco", "fries"] },
  },
  required: ["name", "age"],
};

type Dog = {
  name: string;
  age: number;
  hobbies?: string[];
  favoriteFood?: "pizza" | "taco" | "fries";
};

Both objects carry similar if not exactly the same information. This is a code duplication that can annoy developers and introduce bugs if not properly maintained.

That's when json-schema-to-ts comes to the rescue 💪

FromSchema

The FromSchema method allows infering TS types directly from JSON schemas:

import { FromSchema } from "json-schema-to-ts";

const dogSchema = {
  type: "object",
  properties: {
    name: { type: "string" },
    age: { type: "integer" },
    hobbies: { type: "array", items: { type: "string" } },
    favoriteFood: { enum: ["pizza", "taco", "fries"] },
  },
  required: ["name", "age"],
} as const;

type Dog = FromSchema<typeof dogSchema>; // => Will infer the same type as above

Schemas can even be nested, as long as you don't forget the as const statement:

const catSchema = {
  type: "object",
  properties: {
    name: { type: "string" },
    age: { type: "integer" },
    favoriteThings: { enum: ["playing", "sleeping", "sleepingMore"] },
  },
  required: ["name", "age"],
} as const;

const petSchema = {
  anyOf: [dogSchema, catSchema],
} as const;

type Pet = FromSchema<typeof petSchema>;
// => Will work 🙌

Note: The as const statement is used so that TypeScript takes the schema definition to the word (e.g. true is interpreted as the true constant and not as boolean). It is pure TypeScript and has zero impact on the compiled code.

Docs

Installation

# npm
npm install --save-dev json-schema-to-ts

# yarn
yarn add --dev json-schema-to-ts

Use cases

Litterals

const litteralSchema = {
  type: "null", // "boolean", "string", "integer", "number"
} as const;

type Litteral = FromSchema<typeof litteralSchema>;
// => null, boolean, string or number

Objects

const objectSchema = {
  type: "object",
  properties: {
    foo: { type: "string" },
    bar: { type: "number" },
  },
  required: ["foo"],
} as const;

type Object = FromSchema<typeof objectSchema>;
// => { foo: string, bar?: number }

FromSchema partially supports the use of the additionalProperties and patternProperties keyword:

• Contrary to the specifications, additionalProperties is considered false by default for clearer typings. Set its value to true to signal that additional properties can be used:

const additionalPropertiesSchema = {
  ...objectSchema,
  additionalProperties: true,
} as const;

type Object = FromSchema<typeof additionalPropertiesSchema>;
// => { [x: string]: any; foo: string; bar?: number }

• Used on their own, typed additionalProperties and/or patternProperties are supported:

const typedValuesSchema = {
  type: "object",
  additionalProperties: {
    type: "boolean",
  },
} as const;

type Object = FromSchema<typeof typedValuesSchema>;
// => { [key: string]: boolean }

const patternSchema = {
  type: "object",
  patternProperties: {
    "^S": { type: "string" },
    "^I": { type: "integer" },
  },
} as const;

type Object = FromSchema<typeof patternSchema>;
// => { [key: string]: string | number }

• However, due to TypeScript limitations, when used in combination with the properties keyword, extra properties will always be typed as any to avoid conflicts with base properties.

Arrays

const arraySchema = {
  type: "array",
  items: { type: "string" },
} as const;

type Array = FromSchema<typeof arraySchema>;
// => string[]

Tuples

const tupleSchema = {
  type: "array",
  items: [{ type: "boolean" }, { type: "string" }],
} as const;

type Tuple = FromSchema<typeof tupleSchema>;
// => [] | [boolean] | [boolean, string] | [boolean, string, ...any[]]

FromSchema supports the additionalItems keyword:

• You can deny additional items:

const tupleSchema = {
  type: "array",
  items: [{ type: "boolean" }, { type: "string" }],
  additionalItems: false,
} as const;

type Tuple = FromSchema<typeof tupleSchema>;
// => [] | [boolean] | [boolean, string]

• Or specify a type for additional items:

const tupleSchema = {
  type: "array",
  items: [{ type: "boolean" }, { type: "string" }],
  additionalItems: { type: "number" },
} as const;

type Tuple = FromSchema<typeof tupleSchema>;
// => [] | [boolean] | [boolean, string] | [boolean, string, ...number[]]

FromSchema also supports the minItems and maxItems keyword:

const tupleSchema = {
  type: "array",
  items: [{ type: "boolean" }, { type: "string" }],
  minItems: 1,
  maxItems: 2,
} as const;

type Tuple = FromSchema<typeof tupleSchema>;
// => [boolean] | [boolean, string]

Multiple Types

const multipleTypesSchema = {
  type: ["null", "string"],
} as const;

type Tuple = FromSchema<typeof multipleTypesSchema>;
// => null | string

Other properties like required or additionalItems will also work 🙌

const multipleTypesSchema = {
  type: ["array", "object"],
  items: [{ type: "string" }],
  additionalItems: false,
  properties: {
    name: { type: "string" },
    age: { type: "number" },
  },
  required: ["name"],
} as const;

type Tuple = FromSchema<typeof multipleTypesSchema>;
// => [] | [string] | { name: string, age?: number }

Const

const fooSchema = {
  const: "foo",
} as const;

type Foo = FromSchema<typeof fooSchema>;
// => "foo"

Enums

const enumSchema = {
  enum: [true, 42, { foo: "bar" }],
} as const;

type Enum = FromSchema<typeof enumSchema>;
// => true | 42 | { foo: "bar"}

enum can be used concurrently with type.

const enumSchema = {
  type: "string",
  enum: ["foo", "bar", { foo: "bar" }],
} as const;

type Enum = FromSchema<typeof enumSchema>;
// => "foo" | "bar"

AnyOf

const anyOfSchema = {
  anyOf: [
    { type: "string" },
    {
      type: "array",
      items: { type: "string" },
    },
  ],
} as const;

type AnyOf = FromSchema<typeof fooSchema>;
// => string | string[]

Similar packages

Other packages similar to json-schema-to-ts

typescript-json-schema

The typescript-json-schema package generates JSON Schema from your TypeScript types. It works in the opposite direction compared to json-schema-to-ts, which converts JSON Schema to TypeScript types. This package is useful if you start with TypeScript types and need to generate JSON Schema for validation or documentation purposes.

ajv

Ajv is a JSON Schema validator that can also generate TypeScript types from JSON Schema using additional plugins. While its primary focus is on validation, it offers similar functionality to json-schema-to-ts when combined with the appropriate plugins.

zod

Zod is a TypeScript-first schema declaration and validation library. It allows you to define schemas in TypeScript and infer types from them. While it doesn't convert JSON Schema to TypeScript types directly, it provides a similar type-safe experience for defining and validating data structures.