Async API documentation is used for documenting events in event-driven systems, like Kafka events. All of the event DTOs are stored in one place. It supports YAML and JSON formats.

It contains information about channels and components. Channels and components are defined with their messages and DTO schemas, respectively.

{
  "asyncapi": "2.6.0",
  "info": {
    "title": "Events docs",
    "version": "1.0.0"
  },
  "channels": {
    "topic_name": {
      "publish": {
        "message": {
          "schemaFormat": "application/vnd.oai.openapi;version=3.0.0",
          "payload": {
            "type": "object",
            "properties": {
              "counter": {
                "type": "number"
              }
            },
            "required": ["counter"]
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "EventDto": {
        "type": "object",
        "properties": {
          "counter": {
            "type": "number"
          }
        },
        "required": ["counter"]
      }
    }
  }
}

Autogeneration

Async API docs can be autogenerated by following multiple steps:

• define DTOs and their required and optional fields with ApiProperty and ApiPropertyOptional decorators (from the @nestjs/swagger package), respectively
• generate OpenAPI docs from the defined DTOs
• parse and reuse component schemas from generated OpenAPI documentation to build channel messages and component schemas for Async API docs

Validation

Use AsyncAPI Studio to validate the written specification.

Preview

There are multiple options

• AsyncAPI Studio

• VSCode extension asyncapi-preview, open the command palette, and run the Preview AsyncAPI command.

UI generation

• Install @asyncapi/cli and corresponding template package (e.g., @asyncapi/html-template, @asyncapi/markdown-template)
• Update package.json with scripts

{
  "scripts": {
    // ...
    "generate-docs:html": "asyncapi generate fromTemplate ./asyncapi/asyncapi.json @asyncapi/html-template --output ./docs/html",
    "generate-docs:markdown": "asyncapi generate fromTemplate ./asyncapi/asyncapi.json @asyncapi/markdown-template --output ./docs/markdown"
  }
}

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OpenAPI is a language-agnostic specification for declaring API documentation for REST APIs. It contains the following information:

• API information like title, description, version
• endpoints definitions with request and response parameters
• DTOs and security schemas

openapi: 3.0.0
paths:
  /users:
    post:
      operationId: UsersController_createUser
      summary: Create user
      description: Create a new user
      parameters: []
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/CreateUserDto'
      responses:
        '201':
          description: 'User is created'
info:
  title: nestjs-starter
  description: Minimal NestJS boilerplate
  version: 0.1.0
  contact: {}
tags: []
servers: []
components:
  securitySchemes:
    token:
      type: apiKey
      scheme: api_key
      in: header
      name: auth-token
  schemas:
    CreateUserDto:
      type: object
      properties:
        firstName:
          type: string
          example: tester
          description: first name of the user
      required:
        - firstName

NestJS provides a Swagger plugin for generating the API docs.

Setup

Configure API documentation with the specified endpoint, like /api-docs, which shows the generated docs.

const SWAGGER_API_ENDPOINT = '/api-docs';
// ...
export const setupApiDocs = (app: INestApplication): void => {
  const options = new DocumentBuilder()
    .setTitle(SWAGGER_API_TITLE)
    .setDescription(SWAGGER_API_DESCRIPTION)
    .setVersion(SWAGGER_API_VERSION)
    .addSecurity('token', {
      type: 'apiKey',
      scheme: 'api_key',
      in: 'header',
      name: 'auth-token'
    })
    .addBearerAuth()
    .build();
  const document = SwaggerModule.createDocument(app, options);
  SwaggerModule.setup(SWAGGER_API_ENDPOINT, app, document);
};

Configure the plugin in the NestJS config file.

{
  "compilerOptions": {
    "plugins": ["@nestjs/swagger"]
  }
}

JSON and YAML formats are generated at /api-docs-json and /api-docs-yaml endpoints, respectively.

Decorators

• ApiTags groups endpoints

@ApiTags('users')
@Controller('users')
export class UsersController {
  // ...
}

• ApiOperation provides more details like a summary and description of the endpoint

@ApiOperation({
  summary: 'Get user',
  description: 'Get user by id',
})
@Get(':id')
async getById(
  @Param('id', new ParseUUIDPipe()) id: string,
): Promise<UserDto> {
// ...
}

• ApiOperation can be used to mark an endpoint as deprecated

@ApiOperation({ deprecated: true })

• @ApiProperty and @ApiPropertyOptional should be used for request and response DTOs fields. Example and description values will be shown in the generated documentation.

export class CreateUserDto {
  @ApiProperty({ example: 'John', description: 'first name of the user' })
  // ...
  public firstName: string;
  @ApiPropertyOptional({ example: 'Doe', description: 'last name of the user' })
  // ...
  public lastName?: string;
}

• ApiHeader documents endpoint headers

@ApiHeader({
  name: 'correlation-id',
  required: false,
  description: 'unique id for correlated logs',
  example: '7ea2c7f7-8b46-475d-86f8-7aaaa9e4a35b',
})
@Get()
getHello(): string {
// ...
}

• ApiResponse specifies which responses are expected, like error responses. NestJS' Swagger package provides decorators for specific status codes like ApiBadRequestResponse.

// ...
@ApiResponse({ type: NotFoundException, status: HttpStatus.NOT_FOUND })
@ApiBadRequestResponse({ type: BadRequestException })
@Get(':id')
async getById(
  @Param('id', new ParseUUIDPipe()) id: string,
): Promise<UserDto> {
  return this.userService.findById(id);
}
// ...

• ApiSecurity('token') uses a custom-defined security strategy, token in this case. Other options are to use already defined strategies like ApiBearerAuth.

@ApiSecurity('token')
@Controller()
export class AppController {
  // ...
}
// ...
@ApiBearerAuth()
@Controller()
export class AppController {
  // ...
}

• ApiExcludeEndpoint and ApiExcludeController exclude one endpoint and the whole controller, respectively.

export class AppController {
  @ApiExcludeEndpoint()
  @Get()
  getHello(): string {
    // ...
  }
}
// ...
@ApiExcludeController()
@Controller()
export class AppController {
  // ...
}

• ApiBody with ApiExtraModels add an example for the request body

const CreateUserDtoExample = {
    firstName: 'Tester',
  };
  @ApiExtraModels(CreateUserDto)
  @ApiBody({
    schema: {
      oneOf: refs(CreateUserDto),
      example: CreateUserDtoExample,
    },
  })
  @Post()
  async createUser(@Body() newUser: CreateUserDto): Promise<UserDto> {
    // ...
  }

Importing API to Postman

Import JSON version of API docs as Postman API with Import → Link option (e.g., URL http://localhost:8081/api-docs-json). Imported API collection will be available in the APIs tab.

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JSDoc provides adding types to the JavaScript codebase with appropriate conventions inside comments so different IDEs like Visual Studio Code can recognize defined types, show them and make coding easier with auto-completion. Definitions are put inside /** */ comments.

Examples

Custom types can be defined with @typedef and @property tags. Every property has a type and if the property is optional, its name is put between square brackets, and a description can be included after the property name. Global types should be defined in *.jsdoc.js files so they can be used in multiple files without importing. * represents any type.

/**
 * @typedef {object} CollectionItem
 * @property {string} [collectionName] - collection name is optional string field
 * @property {boolean} isRevealed - reveal status
 * @property {number} floorPrice - floor price
 * @property {?string} username - username is a nullable field
 * @property {Array.<number>} prices - prices array
 * @property {Array.<string>} [buyers] - optional buyers array
 * @property {Array.<Object<string, *>>} data - some data
 */

Classes are auto recognized so @class and @constructor tags can be omitted.

/**
 * Scraper for websites
 */
class Scraper {
  /**
   * Create scraper
   * @param {string} url - website's URL
   */
  constructor(url) {
    this.url = url;
  }
  // ...
}

Comments starting with the description can omit @description tag. Function parameters and function return types can be defined with @param and @returns tags. Multiple return types can be handled with | operator. Deprecated parts of the codebase can be annotated with @deprecated tag.

/**
 * Gets prices list
 * @private
 * @param {Array.<number>} prices - prices array
 * @returns {string|undefined}
 */
const getPricesList = (prices) => {
  if (prices.length > 0) return prices.join(',');
};
/**
 * Get data from the API
 * @deprecated
 * @returns {Promise<CollectionItem>}
 */
const getData = async () => {
  // ...
};

Variable types can be documented with @type tags and constants can utilize @const tags.

/**
 * Counter for the requests
 * @type {number}
 */
let counter;
/**
 * HTTP timeout in milliseconds
 * @const {number}
 */
const HTTP_TIMEOUT_MS = 3000;

Enums can be documented with @enum and @readonly tags.

/**
 * Some states
 * @readonly
 * @enum {string}
 */
const state = {
  STARTED: 'STARTED',
  IN_PROGRESS: 'IN_PROGRESS',
  FINISHED: 'FINISHED'
};

Docs validation

Linter can validate the docs. Add the following package and update the linter configuration file.

npm i -D eslint-plugin-jsdoc

// .eslintrc.js
module.exports = {
  extends: ['plugin:jsdoc/recommended']
};

Run the linter and it will show warnings if something has to be improved.

Generating the docs overview

Run the following command to recursively generate the HTML files with the docs overview, including the README.md and package.json content. Symbols marked with @private tags will be skipped.

npx jsdoc src -r --destination docs --readme ./README.md --package ./package.json

This command can be included in the CI/CD pipeline, it depends on the needs of the project.

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