ng-openapi-gen: An OpenAPI 3 code generator for Angular

This project is a NPM module that generates model interfaces and web service clients from an OpenApi 3 specification. The generated classes follow the principles of Angular. The generated code is compatible with Angular 12+.

For a generator for Swagger 2.0, use ng-swagger-gen instead.

Highlights

• It should be easy to use and to integrate with Angular CLI;
• It should support OpenAPI specifications in both JSON and YAML formats;
• Each tag in the OpenAPI specification generates an Angular @Injectable() service;
• An Angular @NgModule() is generated, which provides all services;
• It should be easy to access the original HttpResponse, for example, to read headers. This is achieved by generating a variant suffixed with $Response for each generated method;
• OpenAPI supports combinations of request body and response content types. For each combination, a distinct method is generated;
• It should be possible to specify a subset of services to generate. Only the models actually used by that subset should be generated;
• It should be easy to specify a root URL for the web service endpoints;
• Generated files should compile using strict TypeScript compiler flags, such as noUnusedLocals and noUnusedParameters;
• For large APIs it is possible to generate only functions for each API operation, and not entire services. This allows for tree-shakable code to be generated, resulting in lower bundle sizes.

Limitations

• Only standard OpenAPI 3 descriptions will be generated. ng-swagger-gen allows several extensions, specially types from JSON schema, but they are out of scope for ng-openapi-gen. There is, however, support for 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;

Relationship with ng-swagger-gen

This project uses the same philosophy as ng-swagger-gen, and was built by the same team. We've learned a lot with ng-swagger-gen and have applied all the acquired knowledge to build ng-openapi-gen.

There were several reasons to not build a new major version of ng-swagger-gen that supports OpenAPI 3, but instead, to create a new project. The main differences between ng-openapi-gen and ng-swagger-gen are:

• The first, more obvious and more important is the specification version, OpenAPI 3 vs Swagger 2;
• The generator itself is written in TypeScript, which should be easier to maintain;
• There is an extensive test suite for the generator;
• The command-line arguments are more robust, derived directly from the JSON schema definition for the configuration file, easily allowing to override any specific configuration on CLI.
• Root enumerations (schemas of type = string | number | integer) can be generated as TypeScript's enum's. This is enabled by default. Inline enums are not, because it would require another type to be exported in the container type.

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 the 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.

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": "out/person-place",
  "ignoreUnusedModels": false
}

Specifying the root URL / web service endpoint

The easiest way to specify a custom root URL (web service endpoint URL) is to use forRoot method of ApiModule and set the rootUrl property from there.

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    HttpClientModule,
    ApiModule.forRoot({ rootUrl: 'https://www.example.com/api' }),
  ],
  bootstrap: [
    AppComponent
  ]
})
export class AppModule { }

Alternatively, you can inject the ApiConfiguration instance in some service or component, such as the AppComponent and set the rootUrl property there.

Using functional API calls

Starting with version 0.50.0, ng-openapi-gen generates a function with the implementation of each actual API call. The generated services delegate to such functions.

However, it is possible to disable the entire services generation, which will avoid the need to include all such services in the application. As a result, the application will be more tree-shakable, resulting in smaller bundle sizes. This is specially true for large 3rd party APIs, in which, for example, a single service (OpenAPI tag) has many methods, but only a few are actually used. Combined with the option "enumStyle": "alias", the footprint of the API generation will be minimal.

Each generated function receives the following arguments:

• Angular's HttpClient instance;
• The API rootUrl (the operation knowns the relative URL, and will use this root URL to build the full endpoint path);
• The actual operation parameters. If it has no parameters or all parameters are optional, the params option will be optional as well;
• The optional http context.

Clients can directly call the function providing the given parameters. However, to make the process smoother, it is also possible to generate a general service specifically to invoke such functions. Its generation is disabled by default, but can be enabled by setting the option "apiService": "ApiService" (or another name your prefer). With this, a single @Injectable service is generated. It will provide the functions with the HttpClient and rootUrl (from ApiConfiguration).

It then provides 2 methods for invoking the functions:

• invoke: Calls the function and returns the response body;
• invoke$Response: Calls the function and returns the entire response, so additional metadata can be read, such as status code or headers.

Here is an example class using the ApiService:

import { Directive, OnInit, inject } from '@angular/core';
import { ApiService } from 'src/api/api.service';
import { getResults } from 'src/api/fn/api/get-results';
import { Result } from 'src/api/models';
import { Observable } from 'rxjs';

@Directive()
export class ApiFnComponent implements OnInit {
  results$!: Observable<Result[]>;

  apiService = inject(ApiService);

  ngOnInit() {
    // getResults is the operation function. The second argument is the actual parameters passed to the function
    this.results$ = this.apiService.invoke(getResults, { limit: 10 });
  }
}

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 HttpInterceptor should be used. It is basically an @Injectable that is called before each request, and can customize both requests and responses.

Here is an example:

@Injectable()
export class ApiInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    // Apply the headers
    req = req.clone({
      setHeaders: {
        'ApiToken': '1234567890'
      }
    });

    // Also handle errors globally
    return next.handle(req).pipe(
      tap(x => x, err => {
        // Handle this err
        console.error(`Error performing request, status code = ${err.status}`);
      })
    );
  }
}

Then, both the HttpInterceptor implementation and the injection token HTTP_INTERCEPTORS pointing to it must be provided in your application module, like this:

import { NgModule, Provider, forwardRef } from '@angular/core';
import { HTTP_INTERCEPTORS } from '@angular/common/http';

import { ApiInterceptor } from './api.interceptor';

export const API_INTERCEPTOR_PROVIDER: Provider = {
  provide: HTTP_INTERCEPTORS,
  useExisting: forwardRef(() => ApiInterceptor),
  multi: true
};

@NgModule({
  providers: [
    ApiInterceptor,
    API_INTERCEPTOR_PROVIDER
  ]
})
export class AppModule {}

Finer control over specific requests can also be achieved, such as:

• Set the immediate next request to use a BASIC authentication for login, and the subsequent ones to use a session key in another request header;
• Set the next request to not use the default error handling, and handle errors directly in the calling code.

To do so, just create another shared @Injectable(), for example, called ApiRequestConfiguration, which has state for such special cases. Then inject it on both the HttpInterceptor and in the client code that makes requests. Here is an example for such class for controlling the authentication:

import { Injectable } from '@angular/core';
import { HttpRequest } from '@angular/common/http';

/**
 * Configuration for the performed HTTP requests
 */
@Injectable()
export class ApiRequestConfiguration {
  private nextAuthHeader: string;
  private nextAuthValue: string;

  /** Set to basic authentication */
  basic(user: string, password: string): void {
    this.nextAuthHeader = 'Authorization';
    this.nextAuthValue = 'Basic ' + btoa(user + ':' + password);
  }

  /** Set to session key */
  session(sessionKey: string): void {
    this.nextAuthHeader = 'Session';
    this.nextAuthValue = sessionKey;
  }

  /** Clear any authentication headers (to be called after logout) */
  clear(): void {
    this.nextAuthHeader = null;
    this.nextAuthValue = null;
  }

  /** Apply the current authorization headers to the given request */
  apply(req: HttpRequest<any>): HttpRequest<any> {
    const headers = {};
    if (this.nextAuthHeader) {
      headers[this.nextAuthHeader] = this.nextAuthValue;
    }
    // Apply the headers to the request
    return req.clone({
      setHeaders: headers
    });
  }
}

Then change the ApiInterceptor class to call the apply method. And, of course, add ApiRequestConfiguration to your module providers and inject it on your components or services.

Setting up a node script

Regardless If your Angular project was generated or is managed by Angular CLI, or you have started your project with some other seed (for example, using webpack directly), you can setup a script to make sure the generated API classes are consistent with the OpenAPI descriptor.

It is not a good practice to have generated 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.

Setting up a script will make sure 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": {
    "ng-openapi-gen": "ng-openapi-gen",
    "start": "npm run ng-openapi-gen && npm run ng -- serve",
    "build": "npm run ng-openapi-gen && npm run ng -- build -prod"
  }
}

This way whenever you run npm start or npm run build, the API classes will be generated before actually serving / building your application.

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

{
  "scripts": {
    "ng-openapi-gen": "ng-openapi-gen",
    "generate.api1": "npm run ng-openapi-gen -c api1.json",
    "generate.api2": "npm run ng-openapi-gen -c api2.json",
    "generate": "npm run generate.api1 && npm run generate.api2",
    "start": "npm run generate && npm run ng -- serve",
    "build": "npm run generate && 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 0.17.2 version, subject to change in the future):

{{^hasSuperClasses}}import { BaseModel } from
'app/base-model';{{/hasSuperClasses}}

export interface {{typeName}}
{{#hasSuperClasses}} extends {{#superClasses}}{{{.}}}{{^@last}},
{{/@last}}{{/superClasses}}{{/hasSuperClasses}}
{{^hasSuperClasses}} extends BaseModel{{/hasSuperClasses}}
{
{{#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 recieves 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.

On the other hand, for developing / running tests, jasmine-ts is used, so the tests run directly from TypeScript. There's even a committed VisualStudio Code debug configuration for tests.

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.