json-schema-typescript-generator

Generate typescript files from json schema files

Contents

Install

Usage

Typescript

JSONSchema

Approach

Install

Install via one of the commands

npm -i json-schema-typescript-generator -D
yarn add json-schema-typescript-generator -D

Usage

To generate .ts files, invoke the main function with an Options object like so

import { main, Options } from 'json-schema-typescript-generator';

const options: Options = {...}
main(options);

The Options object is defined as follows:

{
  files: {
    cwd: string;
    source: {
      dir: string;
      encoding: BufferEncoding;
      recursive: boolean;
    };
    destination: {
      dir: string;
      preClean: boolean;
      indexFiles: boolean;
    };
  };
  ts: {
    optionalFields: OptionalFieldPattern;
    untyped: UntypedType;
  };
}

All options are optional and fall back to their defaults if not given, which are as follows:

{
  files: {
    cwd: process.cwd(),
    source: {
      dir: 'src/schemas',
      encoding: 'utf-8',
      recursive: true
    },
    destination: {
      dir: 'src/generated',
      preClean: false,
      indexFiles: true
    }
  },
  ts: {
    optionalFields: OptionalFieldPattern.QUESTION,
    untyped: UntypedType.UNKNOWN
  }
}

The option

options.files.destination.preClean

defines whether the destination folder will be deleted before generating typescript files.

The option

options.files.destination.indexFiles

defines whether index.ts files will be generated in each destination folder.

Note that the folders given by the options

options.files.source.dir
options.files.destination.dir

will be resolved relative to the folder given by the option

options.files.cwd

Typescript

There are 2 options which define the style of code generated

OptionalFieldPattern

The option

options.ts.optionalFields

defines how optional fields are represented and can take 1 of 2 values:

OptionalFieldPattern.QUESTION

type Example = {
  a?: string;
};

or

OptionalFieldPattern.PIPE_UNDEFINED

type Example = {
  a: string | undefined;
}

UntypedType

The option

options.ts.untyped

defines the fallback type used for types that failed to be generated, eg. perhaps the schema was empty, or an id was missing or a typo was made in a $ref entry etc. It can take 1 of 4 values:

UntypedType.ANY

type Example = {
  a: any
}

or

UntypedType.NEVER

type Example = {
  a: never
}

or

UntypedType.UNDEFINED

type Example = {
  a: undefined
}

or

UntypedType.UNKNOWN

type Example = {
  a: unknown
}

JSONSchema

Support for properties defined in the JSON Schema are as follows:

Key	Support	Notes
$id	✔
$schema	✘	Action in TODO to support specific schema versions
$ref	✔	local definition absolute reference to root/inner definition relative reference to root/inner definition
enum	✔	null booleans numbers strings
type	✔	"null" "boolean" "integer" "number" "string" "array" "object" Or an array of the above
number properties	✘	multipleOf minimum maximum exclusiveMinimum exclusiveMaximum No typescript support for multipleOf Open question on GitHub about number ranges
string properties	✘	minLength maxLength pattern format Typescript support for string patterns and formats in v4.1 Typescript's implementation produces a union of every possible combination so is not suitable for patterns or formats
array properties	✔	items uniqueItems additionalItems T[] - Array if items is a schema and uniqueItems=false Set<T> - Set if items is a schema and uniqueItems=true [T, U, V] - Tuple if items is an array
array properties	✘	contains minItems maxItems array (and possibly tuple) min length: type MinLengthArray<T> = [T, T, ...T[]]; Although no typescript support for a Set<T> of specific size No typescript support for contains
object properties	✔	properties additionalProperties
combinations	✔	allOf anyOf oneOf If oneOf is used, OneOf_n types are dynamically generated in files that need them, for arrays with many items these types can get large and will be updated if typescript adds native support

Approach

The approach this package takes is to only do one thing but do it well, ie. transforming schema files to typescript files. It doesn't download any schemas, or do any validation, consistency checking, linting, prettifying etc. It assumes the schema author knows exactly what they want and it will generate typescript files that represent the schemas given as closely as possible, even if the generated types don't make sense or cannot be satisfied.

An example will make this clear:

Given the following schema in a file called A.json

{
  "type": "number",
  "$ref": "#/definitions/BOOL",
  "enum": [
    null,
    "tuv",
    "xyz"
  ],
  "definitions": {
    "BOOL": {
      "type": "boolean"
    }
  }
}

Invoking the generator will generate a file A.ts containing:

export type A = BOOL
& (null | 'tuv' | 'xyz')
& number;

export type BOOL = boolean;

Clearly type A cannot ever be satisfied, but it does match what the schema specified