OpenAPI Typescript Codegen

> Node.js library that generates Typescript clients based on the OpenAPI specification.

Why?

• Frontend ❤️ OpenAPI, but we do not want to use JAVA codegen in our builds
• Quick, lightweight, robust and framework agnostic 🚀
• Supports generation of TypeScript clients
• Supports generations of fetch and XHR http clients
• Supports OpenAPI specification v2.0 and v3.0
• Supports JSON and YAML files for input
• Supports generation through CLI, Node.js and NPX
• Supports tsc and @babel/plugin-transform-typescript
• Supports external references using json-schema-ref-parser

Install

npm install openapi-typescript-codegen --save-dev

Usage

$ openapi --help

  Usage: openapi [options]

  Options:
    -V, --version             output the version number
    -i, --input        OpenAPI specification, can be a path, url or string content (required)
    -o, --output       Output directory (required)
    -c, --client       HTTP client to generate [fetch, xhr, node] (default: "fetch")
    --useOptions              Use options instead of arguments
    --useUnionTypes           Use union types instead of enums
    --exportCore       Write core files to disk (default: true)
    --exportServices   Write services to disk (default: true)
    --exportModels     Write models to disk (default: true)
    --exportSchemas    Write schemas to disk (default: false)

  Examples
    $ openapi --input ./spec.json
    $ openapi --input ./spec.json --output ./dist
    $ openapi --input ./spec.json --output ./dist --client xhr

Example

package.json

{
    "scripts": {
        "generate": "openapi --input ./spec.json --output ./dist"
    }
}

NPX

npx openapi-typescript-codegen --input ./spec.json --output ./dist

Node.js API

const OpenAPI = require('openapi-typescript-codegen');

OpenAPI.generate({
    input: './spec.json',
    output: './dist'
});

// Or by providing the content of the spec directly 🚀
OpenAPI.generate({
    input: require('./spec.json'),
    output: './dist'
});

Features

Argument style vs. Object style --useOptions

There's no named parameter in JavaScript or TypeScript, because of that, we offer the flag --useOptions to generate code in two different styles.

Argument-style:

function createUser(name: string, password: string, type?: string, address?: string) {
    // ...
}

// Usage
createUser('Jack', '123456', undefined, 'NY US');

Object-style:

function createUser({ name, password, type, address }: {
    name: string,
    password: string,
    type?: string
    address?: string
}) {
    // ...
}

// Usage
createUser({
    name: 'Jack',
    password: '123456',
    address: 'NY US'
});

Enums vs. Union Types --useUnionTypes

The OpenAPI spec allows you to define enums inside the data model. By default, we convert these enums definitions to TypeScript enums. However, these enums are merged inside the namespace of the model, this is unsupported by Babel, see docs. Because we also want to support projects that use Babel @babel/plugin-transform-typescript, we offer the flag --useUnionTypes to generate union types instead of the traditional enums. The difference can be seen below:

Enums:

// Model
export interface Order {
    id?: number;
    quantity?: number;
    status?: Order.status;
}

export namespace Order {
    export enum status {
        PLACED = 'placed',
        APPROVED = 'approved',
        DELIVERED = 'delivered',
    }
}

// Usage
const order: Order = {
    id: 1,
    quantity: 40,
    status: Order.status.PLACED
}

Union Types:

// Model
export interface Order {
    id?: number;
    quantity?: number;
    status?: 'placed' | 'approved' | 'delivered';
}

// Usage
const order: Order = {
    id: 1,
    quantity: 40,
    status: 'placed'
}

Runtime schemas --exportSchemas

By default, the OpenAPI generator only exports interfaces for your models. These interfaces will help you during development, but will not be available in JavaScript during runtime. However, Swagger allows you to define properties that can be useful during runtime, for instance: maxLength of a string or a pattern to match, etc. Let's say we have the following model:

{
    "MyModel": {
        "required": [
            "key",
            "name"
        ],
        "type": "object",
        "properties": {
            "key": {
                "maxLength": 64,
                "pattern": "^[a-zA-Z0-9_]*$",
                "type": "string"
            },
            "name": {
                "maxLength": 255,
                "type": "string"
            },
            "enabled": {
                "type": "boolean",
                "readOnly": true
            },
            "modified": {
                "type": "string",
                "format": "date-time",
                "readOnly": true
            }
        }
    }
}

This will generate the following interface:

export interface MyModel {
    key: string;
    name: string;
    readonly enabled?: boolean;
    readonly modified?: string;
}

The interface does not contain any properties like maxLength or pattern. However, they could be useful if we wanted to create some form where a user could create such a model. In that form you would iterate over the properties to render form fields based on their type and validate the input based on the maxLength or pattern property. This requires us to have this information somewhere... For this we can use the flag --exportSchemas to generate a runtime model next to the normal interface:

export const $MyModel = {
    properties: {
        key: {
            type: 'string',
            isRequired: true,
            maxLength: 64,
            pattern: '^[a-zA-Z0-9_]*$',
        },
        name: {
            type: 'string',
            isRequired: true,
            maxLength: 255,
        },
        enabled: {
            type: 'boolean',
            isReadOnly: true,
        },
        modified: {
            type: 'string',
            isReadOnly: true,
            format: 'date-time',
        },
    },
};

These runtime object are prefixed with a $ character and expose all the interesting attributes of a model and its properties. We can now use this object to generate the form:

import { $MyModel } from './generated';

// Some pseudo code to iterate over the properties and return a form field
// the form field could be some abstract component that renders the correct
// field type and validation rules based on the given input.
const formFields = Object.entries($MyModel.properties).map(([key, value]) => (
    
));

const MyForm = () => (
    <form>
        {formFields}
    </form>
);

Enum with custom names and descriptions

You can use x-enum-varnames and x-enum-descriptions in your spec to generate enum with custom names and descriptions. It's not in official spec yet. But it's a supported extension that can help developers use more meaningful enumerators.

{
    "EnumWithStrings": {
        "description": "This is a simple enum with strings",
        "enum": [
            0,
            1,
            2
        ],
        "x-enum-varnames": [
            "Success",
            "Warning",
            "Error"
        ],
        "x-enum-descriptions": [
            "Used when the status of something is successful",
            "Used when the status of something has a warning",
            "Used when the status of something has an error"
        ]
    }
}

Generated code:

enum EnumWithStrings {
    /*
    * Used when the status of something is successful
    */
    Success = 0,
    /*
    * Used when the status of something has a warning
    */
    Waring = 1,
    /*
    * Used when the status of something has an error
    */
    Error = 2,
}

Nullable in OpenAPI v2

In the OpenAPI v3 spec you can create properties that can be NULL, by providing a nullable: true in your schema. However, the v2 spec does not allow you to do this. You can use the unofficial x-nullable in your specification to generate nullable properties in OpenApi v2.

{
    "ModelWithNullableString": {
        "required": ["requiredProp"],
        "description": "This is a model with one string property",
        "type": "object",
        "properties": {
            "prop": {
                "description": "This is a simple string property",
                "type": "string",
                "x-nullable": true
            },
            "requiredProp": {
                "description": "This is a simple string property",
                "type": "string",
                "x-nullable": true
            }
        }
    }
}

Generated code:

interface ModelWithNullableString {
    prop?: string | null,
    requiredProp: string | null,
}

Authorization

The OpenAPI generator supports Bearer Token authorization. In order to enable the sending of tokens in each request you can set the token using the global OpenAPI configuration:

import { OpenAPI } from './generated';

OpenAPI.TOKEN = 'some-bearer-token';

Alternatively, we also support an async method that provides the token for each request. You can simply assign this method to the same TOKEN property in the global OpenAPI object.

import { OpenAPI } from './generated';

const getToken = async () => {
    // Some code that requests a token...
    return 'SOME_TOKEN';
}

OpenAPI.TOKEN = getToken;

References

Local references to schema definitions (those beginning with #/definitions/schemas/) will be converted to type references to the equivalent, generated top-level type.

The OpenAPI generator also supports external references, which allows you to break down your openapi.yml into multiple sub-files, or incorporate third-party schemas as part of your types to ensure everything is able to be TypeScript generated.

External references may be:

• relative references - references to other files at the same location e.g. { $ref: 'schemas/customer.yml' }

• remote references - fully qualified references to another remote location e.g. { $ref: 'https://myexampledomain.com/schemas/customer_schema.yml' }

  For remote references, both files (when the file is on the current filesystem) and http(s) URLs are supported.

External references may also contain internal paths in the external schema (e.g. schemas/collection.yml#/definitions/schemas/Customer) and back-references to the base openapi file or between files (so that you can reference another schema in the main file as a type of an object or array property, for example).

At start-up, an OpenAPI or Swagger file with external references will be "bundled", so that all external references and back-references will be resolved (but local references preserved).

Compare to other generators

Depending on which swagger generator you use, you will see different output. For instance: Different ways of generating models, services, level of quality, HTTP client, etc. I've compiled a list with the results per area and how they compare against the openapi-typescript-codegen.

Click here to see the comparison

FAQ

Babel support

If you use enums inside your models / definitions then those enums are by default inside a namespace with the same name as your model. This is called declaration merging. However, the @babel/plugin-transform-typescript does not support these namespaces, so if you are using babel in your project please use the --useUnionTypes flag to generate union types instead of traditional enums. More info can be found here: Enums vs. Union Types.

Note: If you are using Babel 7 and Typescript 3.8 (or higher) then you should enable the onlyRemoveTypeImports to ignore any 'type only' imports, see https://babeljs.io/docs/en/babel-preset-typescript#onlyremovetypeimports for more info

module.exports = {
    presets: [
        ['@babel/preset-typescript', {
            onlyRemoveTypeImports: true,
        }],
    ],
};

Node.js support

By default, this library will generate a client that is compatible with the (browser based) fetch API, however this client will not work inside the Node.js environment. If you want to generate a Node.js compatible client then you can specify --client node in the openapi call:

openapi --input ./spec.json --output ./dist --client node

This will generate a client that uses node-fetch internally. However, in order to compile and run this client, you will need to install the node-fetch dependencies:

npm install @types/node-fetch --save-dev
npm install node-fetch --save-dev
npm install form-data --save-dev

In order to compile the project and resolve the imports, you will need to enable the allowSyntheticDefaultImports in your tsconfig.json file.

{
    "allowSyntheticDefaultImports": true
}