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LoopBack Test Lab

A collection of test utilities we use to write LoopBack tests.


Test utilities to help writing LoopBack 4 tests:

  • expect - behavior-driven development (BDD) style assertions
  • sinon
    • test spies: functions recording arguments and other information for all of their calls
    • stubs: functions (spies) with pre-programmed behavior
    • mocks: fake methods (like spies) with pre-programmed behavior (like stubs) as well as pre-programmed expectations
  • Helpers for creating supertest clients for LoopBack applications
  • HTTP request/response stubs for writing tests without a listening HTTP server
  • Swagger/OpenAPI spec validation
  • Test sandbox


npm install --save-dev @loopback/testlab

This package is typically used in tests, save it to devDependencies via --save-dev.

Basic use

import {expect} from '@loopback/testlab';

describe('Basic assertions', => {
  it('asserts equal values', => {
    expect({key: 'value'}).to.deepEqual({key: 'value'});

API documentation

Table of contents:


Should.js configured in “as-function” mode (global Object.prototype is left intact) with an extra chaining word to.


Spies, mocks and stubs. Learn more at


Stub implementation of HTTP Request and Response objects, useful for unit tests.

Besides the API provided by shot module (see API Reference), we provide additional APIs to better support async/await flow control and usage in Express-based code.

There are three primary situations where you can leverage stub objects provided by Shot in your unit tests:

  • Code parsing core HTTP Request
  • Code modifying core HTTP Response, including full request/response handlers
  • Code parsing Express HTTP Request or modifying Express HTTP Response

Test request parsing

Use the factory function stubServerRequest to create a stub request that can be passed to methods expecting core HTTP Request on input.

import {stubServerRequest, expect} from '@loopback/testlab';

describe('parseParams', () => {
  it('parses query string arguments', () => {
    const request = stubServerRequest({
      method: 'GET',
      url: '/api/products?count=10',

    const args = parseParams(request, [
      {name: 'count', in: 'query', type: 'number'},


Test response producers

Use the factory function stubHandlerContext to create request & response stubs and a promise to observe the actual response as received by clients.

import {stubHandlerContext, expect} from '@loopback/testlab';

describe('app REST handler', () => {
  it('returns 404 with JSON body when URL not found', async () => {
    const app = express();
    const context = stubHandlerContext({
      method: 'GET',
      url: '/path-does-not-exist',

    // Invoke Express' request handler with stubbed request/response objects
    app(context.request, context.response);

    // Wait until Express finishes writing the response
    const actualResponse = await context.result;

    // Verify the response seen by clients
      error: {
        statusCode: 404,
        message: 'Not Found',

Test code expecting Express Request or Response

Express modifies core HTTP request and response objects with additional properties and methods, it also cross-links request with response and vice versa. As a result, it’s not possible to create Express Request object without the accompanying Response object.

Use the factory function stubExpressContext to create Express-flavored request & response stubs and a promise to observe the actual response as received by clients.

If your tested function is expecting a request object only:

import {stubExpressContext, expect} from '@loopback/testlab';

describe('operationArgsParser', () => {
  it('parses body parameter', async () => {
    const req = givenRequest({
      url: '/',
      payload: {key: 'value'},

    const spec = givenOperationWithRequestBody({
      description: 'data',
      content: {'application/json': {schema: {type: 'object'}}},
    const route = givenResolvedRoute(spec);

    const args = await parseOperationArgs(req, route);

    expect(args).to.eql([{key: 'value'}]);

  function givenRequest(options?: ShotRequestOptions): Request {
    return stubExpressContext(options).request;

Tests verifying code producing HTTP response can await context.result to receive the response as returned to clients.

import {stubExpressContext, expect} from '@loopback/testlab';

describe('response writer', () => {
  it('writes object result to response as JSON', async () => {
    const context = stubExpressContext();

    writeResultToResponse(context.response, {name: 'Joe'});
    const result = await context.result;



Helper function for skipping tests when a certain condition is met. Use this helper together with it or describe.

skipIf(someCondition, it, 'does something', async () => {
  // the test code

Unfortunately, type inference does not work well for describe, you have to help the compiler to figure out the correct types.

skipIf<[(this: Suite) => void], void>(
  'some suite name',
  () => {
    // define the test cases

Under the hood, skipIf invokes the provided test verb by default (e.g. it). When the provided condition was true, then it calls .skip instead (e.g. it.skip).


Helper function for skipping tests on Travis environment. If you need to skip testing on Travis for any reason, use this helper together with it or describe.

skipOnTravis(it, 'does something when some condition', async () => {
  // the test code

Under the hood, skipOnTravis invokes the provided test verb by default (e.g. it). When the helper detects Travis CI environment variables, then it calls .skip instead (e.g. it.skip).


Helper function to create a supertest client connected to a running RestApplication. It is the responsibility of the caller to ensure that the app is running and to stop the application after all tests are done.

Example use:

import {Client, createRestAppClient} from '@loopback/testlab';

describe('My application', () => {
  app: MyApplication; // extends RestApplication
  client: Client;

  before(() => {
    client = createRestAppClient(app);
  after(() => app.stop());

  it('invokes GET /ping', async () => {
    await client.get('/ping?msg=world').expect(200);


Helper function for generating Travis-friendly host ( This is required because Travis is not able to handle IPv6 addresses.


Async wrapper for making HTTP GET requests.

import {httpGetAsync} from '@loopback/testlab';
const response = await httpGetAsync('');


Async wrapper for making HTTPS GET requests.

import {httpsGetAsync} from '@loopback/testlab';
const response = await httpsGetAsync('');


JSON encoding does not preserve properties that are undefined. As a result, deepEqual checks fail because the expected model value contains these undefined property values, while the actual result returned by REST API does not. Use this function to convert a model instance into a data object as returned by REST API.

import {createClientForHandler, toJSON} from '@loopback/testlab';

it('gets a todo by ID', () => {
  return client
    .expect(200, toJSON(persistedTodo));


Verify that your application API specification is a valid OpenAPI spec document.

import {validateApiSpec} from '@loopback/testlab';
import {RestServer} from '@loopback/rest';

describe('MyApp', () => {
  it('has valid spec', async () => {
    const app = new MyApp();
    const server = await app.getServer(RestServer);
    await validateApiSpec(server.getApiSpec());


An error logger that logs the error only when the HTTP status code is not the expected HTTP status code. This is useful when writing tests for error responses:

  • When we don’t want any error messages printed to the console when the server responds with the expected error and the test passes.

  • When something else goes wrong and the server returns an unexpected error status code, and we do want an error message to be printed to the console so that we have enough information to troubleshoot the failing test.

import {createUnexpectedHttpErrorLogger} from '@loopback/testlab';
import {RestApplication} from '@loopback/rest';

describe('MyApp', () => {
  it('does not log a known 401 error to console', async () => {
    const app = new RestApplication();

    const errorLogger = createUnexpectedHttpErrorLogger(401);
    // binds the custom error logger

    const spec = {
      responses: {
    function throwUnauthorizedError() {
      throw new HttpErrors.Unauthorized('Unauthorized!');

    app.route('get', '/', spec, throwUnauthorizedError);

    await app.start();
    // make `GET /` request, assert that 401 is returned


Many tests need use a temporary directory as the sandbox to mimic a tree of files. The TestSandbox class provides such facilities to create and manage a sandbox on the file system.

Create a sandbox

// Create a sandbox as a unique temporary subdirectory under the rootPath
const sandbox = new TestSandbox(rootPath);
const sandbox = new TestSandbox(rootPath, {subdir: true});

// Create a sandbox in the root path directly
// This is same as the old behavior
const sandbox = new TestSandbox(rootPath, {subdir: false});

// Create a sandbox in the `test1` subdirectory of the root path
const sandbox = new TestSandbox(rootPath, {subdir: 'test1'});

// To access the target directory of a sandbox

Reset a sandbox

All files inside a sandbox will be removed when the sandbox is reset. We also try to remove cache from require.

await sandbox.reset();

Delete a sandbox

Removes all files and mark the sandbox unusable.

await sandbox.delete();

Create a directory

Recursively creates a directory within the sandbox.

await sandbox.mkdir(dir);

Copy a file

Copies a file from src to the TestSandbox. If copying a .js file which has an accompanying file in the src file location, the dest file will have its sourceMappingURL updated to point to the original file as an absolute path so you don’t need to copy the map file.

await sandbox.copyFile(src, dest);

Write a json file

Creates a new file and writes the given data serialized as JSON.

await sandbox.writeJsonFile(dest, data);

Write a file

Creates a new file and writes the given data as a UTF-8-encoded text.

await sandbox.writeFile(dest, data);

For more info about supertest, please refer to supertest



Run npm test from the root folder.


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