json-schema-type-mapper

Caution! Experimental software ahead. Use at your own peril.

Type-level conversion of JSON Schema (draft 7) into TypeScript types: handle JSON Schemas as TypeScript types without code generation.

import { Schema } from `json-schema-type-mapper`

declare const myString: Schema<{ type: 'string' }>
//      resolves to --> string

Install

yarn add json-schema-type-mapper

Use case

Suppose you're dealing with the following JSON Schema:

interface User {
  type: 'object'
  properties: {
    id: {
      type: 'number'
    }
    name: {
      type: 'string'
    }
  }
  required: ['id']
  additionalProperties: false
}

And a function whose input conforms to that schema:

function saveUser(user: any) {
  // ...
}

The question is, how would you define user parameter's type? There's a couple of ways to go about it. Either

• a) manually write out a TypeScript type definition that matches the above JSON Schema, or
• b) use a code generation tool such as json-schema-to-typescript to convert the JSON Schema to a TypeScript interface.

I'd probably go the manual route in a simple case like this and opt for code generation with more complex schemas. This library, however, provides a bit of a middle ground between the two by leveraging the type system:

import { Schema } from 'json-schema-type-mapper'

function saveUser(user: Schema<User>) {
  // ...
}

Now user's type resolves to { id: number; name?: string }. We get automatic conversion from JSON Schema to TypeScript, all by leveraging the type system.

Compared to code generation, this method has a number of limitations resulting from the type system's limitations. We get pretty far though, which in itself is testament to the impressive capabilities of TypeScript's type system.

For a thorough list of supported features and examples check the test file.

Complex example

type User = Schema<{
  definitions: {
    address: {
      $id: '#address'
      properties: {
        city: { type: 'string' }
        country: { type: 'string'; enum: ['FI', 'SV', 'NO'] }
      }
      required: ['city', 'country']
    }
  }
  allOf: [
    { $ref: '#address' },
    {
      type: 'object'
      properties: {
        id: {
          anyOf: [
            { type: 'number' },
            { type: 'array'; items: { type: 'number' } }
          ]
        }
        name: {
          type: ['string', 'null']
        }
      }
      required: ['name']
    }
  ]
}>

// resolves to:

type User = {
  [x: string]: JSONValue
  name: string | null
  id?: number | number[] | undefined
  city: string
  country: "FI" | "SV" | "NO"
}

TODO

• [ ] define schema as variable (const schema = { ... } as const) instead of interface
  • requires handling something like DeepReadonly<JSONSchema> since as const turns everything readonly
• [ ] object dependencies
  • [ ] property dependencies
  • [ ] schema dependencies
• [x] allOf, anyOf
• [ ] if, then, else
• [x] $id

Unsupported

• path references like { "$ref": "#/definitions/User" }; consider using $ids instead!
• not only works on primitive types such as { "not": { "type": ["number", "string"] } }
• object propertyNames
• object minProperties/maxProperties
• object patternProperties
• tuple items: [...] limited to a maximum of 6 items for now
• array contains
• array minItems/maxItems
  • hardcoding might work when list length is below a reasonable number...?
• array uniqueness
  • maybe feasible for primitives, not so much for objects?
• oneOf

Related projects

• as-typed with a similar effort of type-level Schema-to-Typescript conversion
• json-schema-typed provides TypeScript definitions for JSON Schema objects