ng-openapi-gen: An OpenAPI 3.0 and 3.1 code generator for Angular

This project is a NPM module that generates model interfaces and web service clients from an OpenApi 3.0 or 3.1 specification. The generated classes follow the principles of Angular. The generated code is compatible with Angular 16+. Support for OpenAPI 3.1 was added since ng-openapi-gen 1.0.

For a generator for Swagger / OpenAPI 2.0, use the ng-swagger-gen instead. Note that ng-swagger-gen has been unmaintained for quite a long time.

Highlights

• Easy to use and to integrate with Angular CLI;
• Supports OpenAPI 3.0 and 3.1 specifications in both JSON and YAML formats;
• Each OpenAPI path is mapped to a function. Those functions are invoked using a generated @Injectable service;
  • Alternatively, it is possible to generate an @Injectable service per tag. This has a bit cleaner API at expense of extra bundle size;
• Supports both Promise (default) and Observable as result types for services;
• Allows accessing the original HttpResponse, for example, to read headers;
  • This is achieved by generating a variant suffixed with $Response on services;
• OpenAPI supports combinations of request body and response content types. For each combination, a distinct function is generated;
• It is possible to specify a subset of functions / models to generate.
  • Think of this not for saving on bundle size, as tree-shaking includes only used functions / models, but to have a cleaner library;
• It is possible to specify a custom root URL for the web service endpoints;
• Generated files should compile using strict TypeScript compiler flags, such as noUnusedLocals and noUnusedParameters.

Limitations

• Only standard OpenAPI 3.0 / 3.1 descriptions will be generated;
  • However, ng-openapi-gen supports a few vendor extensions;
• Servers per operation are not supported;
• Only the first server is used as a default root URL in the configuration;
• No data transformation is ever performed before sending / after returning data;
  • This means that a property of type string and format date-time will always be generated as string, not Date. Otherwise every API call would need to have a processing that would traverse the returned object graph before sending the request to replace all date properties by Date. The same applies to sent requests. Such operations are out of scope for ng-openapi-gen;

Migrating from previous versions to 1.0+

Starting with version 1.0, ng-openapi-gen has updated some default configuration options to better align with current standards. These are the settings that have changed:

• "module": false: Previously, the default was ApiModule. NgModules are no longer needed since standalone components were introduced in Angular 14. All generated @Injectable classes are provided in root module, so we don't need another one.
• "services": false: Previously, the default was true. For some time already, ng-openapi-gen has generated functions for each API operation, and services (one per API tag) are just wrappers around those functions. As services reference all functions, for larger APIs, the bundle size is impacted, because the code for handling all functions in the tag will be bundled, even when using a single one.
• "apiService": "Api". Previously empty, the Api service was not generated, because the default was to use a service per tag. But now we need it to invoke the generated API functions.
• "enumStyle": "alias". Previously, the default was pascal. With this change, by default we'll no longer generate TypeScript enum. Instead, a type is defined with an union of possible values. All other options end up generating a TypeScript enum, which emit a JavaScript class, taking up space in the bundle size.
• "enumArray": true. The major drawback of enumStyle: alias is there's no way to iterate all existing values. With enumArray we generate a sibling .ts file which exports an array (of the correct enum type) with all items on it.
• "promises": true: By default, generated services return Promises, not Observables. If you prefer to keep working with Observables, set "promises": false.

So, if you're upgrading from previous versions and want the generation to be similar, set all these settings in your configuration with their corresponding previous values.

Installing and running

You may want to install ng-openapi-gen globally or just on your project. Here is an example for a global setup:

$ npm install -g ng-openapi-gen
$ ng-openapi-gen --input my-api.yaml --output my-app/src/app/api

Alternatively you can use the generator directly from within your build-script:

import $RefParser from 'json-schema-ref-parser';
import { NgOpenApiGen } from 'ng-openapi-gen';

const options = {
  input: "my-api.json",
  output: "my-app/src/app/api",
}

// load the openapi-spec and resolve all $refs
const RefParser = new $RefParser();
const openApi = await RefParser.bundle(options.input, {
  dereference: { circular: false }
});

const ngOpenGen = new NgOpenApiGen(openApi, options);
ngOpenGen.generate();

This will expect the file my-api.yaml (or my-api.json) to be in the current directory, and will generate files on my-app/src/app/api.

Configuration file and CLI arguments

If the file ng-openapi-gen.json exists in the current directory, it will be read. Alternatively, you can run ng-openapi-gen --config my-config.json (could also be -c) to specify a different configuration file, or even specify the input / output as ng-openapi-gen -i input.yaml or ng-openapi-gen -i input.yaml -o /tmp/generation. The only required configuration property is input, which specified the OpenAPI specification file. The default output is src/app/api.

You can even generate code for multiple APIs in a single project, each with its own configuration file. In this case you'll also probably want to customize names, like having a different configuration and apiService for each API.

For a list with all possible configuration options, see the JSON schema file. You can also run ng-openapi-gen --help to see all available options. Each option in the JSON schema can be passed in as a CLI argument, both in camel case, like --includeTags tag1,tag2,tag3, or in kebab case, like --exclude-tags tag1,tag2,tag3.

Here is an example of a configuration file:

{
  "$schema": "node_modules/ng-openapi-gen/ng-openapi-gen-schema.json",
  "input": "my-file.json",
  "output": "my-app/src/app/api",
  "ignoreUnusedModels": false
}

Using functional API calls

ng-openapi-gen generates a function with the implementation of each actual API call. By default since version 1.0, services per API tag are not generated. To use these functions, a generated @Injectable Api is provided. This name can be changed with the apiService configuration. Here is an example:

import { Component, inject, OnInit, signal } from '@angular/core';
import { RouterOutlet } from '@angular/router';
import { Api } from './api/api';
import { getResults } from './api/fn/operations/get-results';
import { Result } from './api/models';

@Component({
  selector: 'app-root',
  imports: [RouterOutlet],
  templateUrl: './app.html',
  styleUrl: './app.css',
})
export class App implements OnInit {
  protected readonly results = signal<Result[] | null>(null);

  private api = inject(Api);

  async ngOnInit() {
    this.results.set(await this.api.invoke(getResults, { limit: 5 }));
  }
}

Alternatively, ng-openapi-gen can be configured to generate services for each API tag. This was the default before version 1.0. Services provide a slightly cleaner API, at expense of additional bundle size. For larger APIs, suppose your tag has 50 operations and you inject the service in a component, with functions, only the corresponding function will be bundled together with the component code. However, if it injects a service, all 50 functions will be bundled. You can set the "services": true configuration option, and use it like this:

import { Component, inject, OnInit, signal } from '@angular/core';
import { RouterOutlet } from '@angular/router';
import { Result } from './api/models';
import { ResultsService } from './api/services';

@Component({
  selector: 'app-root',
  imports: [RouterOutlet],
  templateUrl: './app.html',
  styleUrl: './app.css',
})
export class App implements OnInit {
  protected readonly results = signal<Result[] | null>(null);

  private resultsService = inject(ResultsService);

  async ngOnInit() {
    this.results.set(await this.resultsService.getResults({ limit: 5 }));
  }
}

Notice there are minimal cosmetic improvements, at expense of extra bundle sizes, especially for large APIs.

Specifying the root URL / web service endpoint

By default, the server specified in the OpenAPI specification is used as root URL for API paths. However, it is a common requirement to configure this from the application. The easiest way is to inject the ApiConfiguration instance (note it can be renamed with the configuration setting) in your bootstrap component and set it directly:

import { Component, inject, OnInit, signal } from '@angular/core';
import { RouterOutlet } from '@angular/router';
import { ApiConfiguration } from './api/api-configuration';
import { Result } from './api/models';

@Component({
  selector: 'app-root',
  imports: [RouterOutlet],
  templateUrl: './app.html',
  styleUrl: './app.css',
})
export class App implements OnInit {
  protected readonly results = signal<Result[] | null>(null);

  private apiConfiguration = inject(ApiConfiguration);

  async ngOnInit() {
    this.apiConfiguration.rootUrl = 'http://localhost:3000';
  }
}

Alternatively, if you generate an NgModule by setting the module configuration (which isn't recommended since Angular's standalone components, and is disabled in ng-openapi-gen by default), you can use its .forRoot({ rootUrl: 'https://www.my-server.com/api'}) method when importing the module. However, this is only kept for historical reasons, and might be removed in the future.

Passing request headers / customizing the request

To pass request headers, such as authorization or API keys, as well as having a centralized error handling, a standard interceptor should be used. Here is an example of a functional interceptor:

import { HttpInterceptorFn } from '@angular/common/http';

export const API_INTERCEPTOR: HttpInterceptorFn = (req, next) => {
  console.log('Intercepted request:', req);
  return next(req);
};

Then, use it in your app.config.ts:

import { ApplicationConfig } from '@angular/core';
import { provideHttpClient, withInterceptors } from '@angular/common/http';
import { API_INTERCEPTOR } from './api-interceptor';

export const appConfig: ApplicationConfig = {
  providers: [
    // ... others
    provideHttpClient(withInterceptors([API_INTERCEPTOR])),
  ],
};

Setting up a node script

It is not a good practice to have generated code committed to the source control system (such as git). The only exception, is for projects using a third party API definition that never changes, in which ng-openapi-gen is expected to run only once. To ignore the generator output folder to in GIT, assuming the default output folder src/app/api, create the src/app/.gitignore file, with a line being api.

If you use an API definition that can change, setup an NPM script to ensure that whenever your project is started or built, the generated files are consistent with the API definition. To do so, create the ng-openapi-gen.json configuration file and add the following scripts to your package.json:

{
  "scripts": {
    "generate:api": "ng-openapi-gen",
    "start": "npm run generate:api && npm run ng -- serve",
    "build": "npm run generate:api && npm run ng -- build -prod"
  }
}

This way whenever you run npm start or npm run build, the API classes will be re-generated.

Also, if you use several configuration files, you can specify multiple times the call to ng-openapi-gen, like:

{
  "scripts": {
    "generate:api": "npm run generate:api:a && npm run generate:api:b",
    "generate.api:a": "ng-openapi-gen -c api-a.json",
    "generate.api:b": "ng-openapi-gen -c api-b.json",
    "start": "npm run generate:api && npm run ng -- serve",
    "build": "npm run generate:api && npm run ng -- build -prod"
  }
}

Supported vendor extensions

Besides the OpenAPI 3 specification, the following vendor extensions are supported:

• x-operation-name: Defined in LoopBack, this extension can be used in operations to specify the actual method name. The operationId is required to be unique among all tags, but with this extension, a shorter method name can be used per tag (service). Example:

paths:
  /users:
    get:
      tags:
        - Users
      operationId: listUsers
      x-operation-name: list
      # ...
  /places:
    get:
      tags:
        - Places
      operationId: listPlaces
      x-operation-name: list
      # ...

• x-enumNames: Generated by NSwag, this extension allows schemas which are enumerations to customize the enum names. It must be an array with the same length as the actual enum values. Example:

components:
  schemas:
    HttpStatusCode:
      type: integer
      enum:
        - 200
        - 404
        - 500
      x-enumNames:
        - OK
        - NOT_FOUND
        - INTERNAL_SERVER_ERROR

Customizing templates

You can customize the Handlebars templates by copying the desired files from the templates folder (only the ones you need to customize) to some folder in your project, and then reference it in the configuration file.

For example, to make objects extend a base interface, copy the object.handlebars file to your src/templates folder. Then, in ng-openapi-gen.json file, set the following: "templates": "src/templates". Finally, the customized src/templates/object.handlebars would look like the following (based on the 1.0 version, subject to change in the future):

import { MyBaseModel} from 'src/app/my-base-model';

export interface {{typeName}} extends MyBaseModel {
{{#properties}}
{{{tsComments}}}{{{identifier}}}{{^required}}?{{/required}}: {{{type}}};
{{/properties}}
{{#additionalPropertiesType}}

  [key: string]: {{{.}}};
{{/additionalPropertiesType}}
}

Custom Handlebars helpers

You can integrate your own Handlebar helpers for custom templates. To do so simply provide a handlebars.js file in the same directory as your templates that exports a function that receives the Handlebars instance that will be used when generating the code from your templates.

module.exports = function(handlebars) {
  // Adding a custom handlebars helper: loud
  handlebars.registerHelper('loud', function (aString) {
    return aString.toUpperCase()
  });
};

Developing and contributing

The generator itself is written in TypeScript. When building, the code is transpiled to JavaScript in the dist folder. And the dist folder is the one that gets published to NPM. Even to prevent publishing from the wrong path, the package.json file has "private": true, which gets replaced by false in the build process.

The tests, on the other hand, run on vitest and run directly from TypeScript.

After developing the changes, to link the module and test it with other node projects, run the following:

npm run build
cd dist
npm link

At that point, the globally available ng-openapi-gen will be the one compiled to the dist folder.