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Previously: This article continues off from Defining and validating the API.

Smoke test API input/output

Once you confirm that the API specification is valid, it’s time to verify that the application implements the API as you have specified it. The input/output testing described below uses Dredd, specifically hello-world in this section. Concrete sample code of hello-world can be found in the hello-world tutorial repository. Although the sample code includes a validated API spec and fully functional hello-world controller, let’s pretend the controller is completely empty. Try it yourself by cloning the repository from GitHub.

For input/output testing, you are going to create three parts:

  1. Input data definition.
  2. Expected output response definition.
  3. Test code.

Parts one and two are included in the API specification. The input data is given as x-example as follows:

"x-example": "Ted"

The expected output as examples:

"examples": {
  "text/plain": "Hello world Ted."

The Dredd module reserves x-example to set the input parameter. the OpenAPI standard defines the examples object as a map from MIME type to the content value. Here, it’s text/plain MIME type. As you see, they are a pair: When you change the input value x-example, you must change examples value as well.

The complete hello-world API specification is the following:

export const controllerSpec =
  swagger: '2.0',
  basePath: '/',
  info: {
    title: 'LoopBack Application',
    version: '1.0.0',
  "paths": {
    "/helloworld": {
      "get": {
        "x-operation-name": "helloWorld",
        "parameters": [
            "name": "name",
            "in": "query",
            "description": "Your name.",
            "required": false,
            "type": "string",
            "x-example": "Ted"
        "responses": {
          "200": {
            "description": "Returns a hello world with your (optional) name.",
            "examples": {
              "text/plain": "Hello world Ted."

The third piece is the test code. To initialize the test environment, you need to create a Dredd instance specifying the configuration. There are two required fields in the configuration object: server and options.path. localhostAndPort + \'/swagger.json\' is the predefined end point LoopBack 4 uses for the client to access the API specification of the service API.

  async function initEnvironment() {
    // By default, the port is set to 3000.
    const app: Application = new HelloWorldApp();
    const server = app.getServer(RestServer);
    // For testing, you'll let the OS pick an available port by setting
    // RestBindings.PORT to 0.
    // app.start() starts up the HTTP server and binds the acquired port
    // number to RestBindings.PORT.
    await app.start();
    // Get the real port number.
    const port: number = await server.get(RestBindings.PORT);
    const localhostAndPort: string = 'http://localhost:' + port;
    const config: object = {
      server: localhostAndPort, // base path to the end points
      options: {
        level: 'fail', // report 'fail' case only
        silent: false, // false for helpful debugging info
        path: [localhostAndPort + '/swagger.json'], // to download apiSpec from the service
    dredd = new Dredd(config);

Since the specification above includes definition of input data and the expected output, you have all the pieces to write the test code:

describe('Api Spec End Points', () => {
  let dredd: any;

  describe('input/output test', () => {

    it('passes match', done => { Error, stats: object) => {
        if (err) return done(err);
          failures: 0,
          errors: 0,
          skipped: 0,

  async function initEnvironment() {
    // see initEnvironment defined above.

Try running the first test:

$ npm test

  Api Spec End Points
    input/output test
fail: GET /helloworld?name=Ted duration: 26ms
fail: body: Real and expected data does not match.

method: GET
uri: /helloworld?name=Ted
    User-Agent: Dredd/4.3.0 (Darwin 16.7.0; x64)
    Content-Length: 0

    Content-Type: text/plain
Hello world Ted
statusCode: 200

statusCode: 500
    date: Wed, 23 Aug 2017 00:17:48 GMT
    connection: close
    content-length: 0

complete: 0 passing, 1 failing, 0 errors, 0 skipped, 1 total
complete: Tests took 27ms

The test report correctly shows that the input is name=Ted and the expected result is Hello world Ted, but the actual result was statusCode: 500 which does not match the expectation. When the hello-world API is implemented, the result would be something like the following:

$ npm test

  Api Spec End Points
    input/output test
complete: 1 passing, 0 failing, 0 errors, 0 skipped, 1 total
complete: Tests took 21ms

It’s a powerful proposition to use the API specification not only for API declaration but for test case declaration. The discussion so far paves the road to “automated controller wireframe-code generation and test-driven development” based on the OpenAPI standard.

At this point, you are ready to make these tests pass by coding up your business logic.

Please refer to Perform an auto-generated smoke test of your REST API from Testing your application for more details.

Next: Defining your testing strategy