Imposter Mock Engine

Bindings for using the Imposter mock engine in JavaScript/Node.js.

Embed live HTTP mocks within your tests, based on OpenAPI specification files or plain REST APIs.

Usage

const {mocks} = require('@imposter-js/imposter');

// start a mock from an OpenAPI spec file on a specific port
await mocks.builder()
    .withPort(8080)
    .withOpenApiSpec('/path/to/openapi_spec.yaml')
    .start();

// call one of the endpoints defined in the OpenAPI spec
const response = await axios.get('http://localhost:8080/products');

// print JSON returned from the mock
console.log(response.data);

This is just a simple example. Your mocks can have dynamic responses, request validation against an OpenAPI schema, data capture, performance delays etc...

See the sample directory for a Node.js project with many examples.

Quickstart

Imposter is available as an npm package.

Install with npm:

npm install --save-dev @imposter-js/imposter

Or add to your package.json as a dev dependency:

"devDependencies": {
  "@imposter-js/imposter": "*"
}

See available versions on the npm registry

Prerequisites

• Install Imposter CLI. Supports macOS, Linux, Windows.
• Ensure you satisfy the CLI requirements (either Docker installed and running, or a JVM installed).

Examples

See the sample directory for a Node.js project with many examples.

Example with Jest

Here's an example using Jest:

const {mocks} = require('@imposter-js/imposter');

jest.setTimeout(30000);

beforeAll(async () => {
    // path to Imposter config directory
    const configDir = `/path/to/order-api`;

    // start a mock on a specific port
    await mocks.start(configDir, 8080);
});

afterAll(async () => {
    mocks.stopAll();
});

it('places an order', async () => {
    // configure the unit under test
    const orderService = new OrderService('http://localhost:8080/orders');

    // call your unit under test, which invokes the mock
    const confirmation = await orderService.placeOrder('product-05');

    // assert values returned by the mock
    expect(confirmation.total).toEqual(18.00);
});

Example using just an OpenAPI file

Here's an example mock that just uses an OpenAPI file:

// start a mock from a bare OpenAPI spec file
// requests are validated against the spec
const mock = await mocks.builder()
    .withOpenApiSpec('/path/to/pet-names-api.yaml')
    .withRequestValidation()
    .start();

// call the mock
const response = await axios.get(`${mock.baseUrl()}/names`);

// Output: [ 'Fluffy', 'Paws' ]
// This is driven by either the 'examples' property
// in the OpenAPI spec, or the schema of the response.
console.log(response.data);

Example with no config file

Here's an example mock that doesn't require any configuration file:

const builder = mocks.builder().withPlugin('rest');

// add a POST resource with a path parameter
const resource = builder.addResource('/users/{userName}', 'POST');

// capture the userName path parameter from the request
// for later use in the response
resource.captures().fromPath('userName');

// respond with a templated message indicating the user
// was created by name
resource.responds(201)
    .withTemplateData('${request.userName} registered')
    .withHeader('Content-Type', 'text/plain');

// spin it up
const mock = await builder.start();

// call the mock
const response = await axios.post(`${mock.baseUrl()}/users/alice`);

// Output: alice registered
// This will vary dynamically, based on the request.
console.log(response.data);

Documentation

• Imposter user documentation - this is the main documentation
• Mock engine configuration - configuration for different engine types

Features

• run standalone mocks in place of real systems
• turn an OpenAPI/Swagger file into a mock API for testing or QA (even before the real API is built)
• decouple your integration tests from the cloud/various back-end systems and take control of your dependencies
• validate your API requests against an OpenAPI specification

Send dynamic responses:

• Provide mock responses using static files or customise behaviour based on characteristics of the request.
• Power users can control mock responses with JavaScript.

Acknowledgements

This project is only possible thanks to the following:

• Imposter
• Jest