data-mocks

Library to mock local data requests using Fetch and XHR

Why is this library useful?

When developing web applications locally, it is not uncommon to request data from an API. However, the API might not actually exist yet, or we might want to control what the responses are.

Typically, this sort of problem has been solved by either writing a separate mock API service alongside the project (i.e. having a Node service running locally with your application) or creating a development database that replicates staging or production environments. Both of these approaches are heavy and can lead to using incorrect data if schemas are out of sync.

This library aims to allow rapid local development without the dependency of a database or fully implemented APIs.

Setup

Assuming your project is using fetch / XHR for HTTP operations:

• Either npm install data-mocks or yarn add data-mocks
• Optional: extract the scenario from URL using the imported extractScenarioFromLocation() function
  • Or you can just hardcode a string to pass through instead
• Import injectMocks() function into your project with import { injectMocks } from 'data-mocks'
• Create an array of Scenario's you would like to use (see examples)
• Pass array of Scenario's to injectMocks()
• Hooray, all HTTP requests to mocked endpoints will now respond with the mocked data you have specified

Examples

Basic mock injection without scenarios (XHR and Fetch)

import { injectMocks } from 'data-mocks';
import axios from 'axios';

const scenarios = {
  default: [
    {
      url: /login/,
      method: 'POST',
      response: { some: 'good response' },
      responseCode: 200
    },
    {
      url: /some-other-endpoint/,
      method: 'GET',
      response: { another: 'response' },
      responseCode: 200,
      delay: 1000
    },
    {
      url: /endpoint-with-headers/,
      method: 'GET',
      response: { same: 'response' },
      responseHeaders: { token: 'mock-token' },
      responseCode: 200
    }
  ]
};

injectMocks(scenarios);

fetch('http://foo.com/login', { method: 'POST', body: {} }).then(response =>
  console.log(response)
); // resolves with { some: 'good response' } after a 200ms delay

fetch('http://foo.com/some-other-endpoint').then(response =>
  console.log(response)
); // resolves with { another: 'response' } after a 1 second delay

axios.post('http://foo.com/login', {}).then(response => console.log(response)); // resolves with { another: 'response' } after a 200ms delay

axios
  .get('http://foo.com/some-other-endpoint')
  .then(response => console.log(response)); // resolves with { another: 'response' } after a 1 second delay

In this example, we define a default scenario in our scenarios map. The Scenario objects are described by the Scenario interface. We then inject the scenarios into our application via the injectMocks() function. Finally, when we use fetch / XHR to make a request to endpoints that match our scenario objects, the mocked responses are returned.

N.B

In the above example we are using axios as our XHR library of choice. However data-mocks will work with any library that uses XMLHttpRequest under the hood.

Mock injection with scenarios

import { injectMocks, extractScenarioFromLocation } from 'data-mocks';
import axios from 'axios';

const scenarios = {
  default: [
    {
      url: /login/,
      method: 'POST',
      response: { some: 'good response' },
      responseCode: 200
    },
    {
      url: /some-other-endpoint/,
      method: 'GET',
      response: { another: 'response' },
      responseCode: 200,
      delay: 1000
    }
  ],
  failedLogin: [
    {
      url: /login/,
      method: 'POST',
      response: { some: 'bad things happened' },
      responseCode: 401
    }
  ]
};

injectMocks(scenarios, 'failedLogin');
// The above line could be rewritten as:
// const scenario = extractScenarioFromLocation(window.location);
// injectMocks(scenarios, scenario);

fetch('http://foo.com/login', { method: 'POST', body: {} }).then(response =>
  console.log(response)
); // resolves with a 401 after a 200ms delay

fetch('http://foo.com/some-other-endpoint').then(response =>
  console.log(response)
); // resolves with { another: 'response' } after a 1 second delay

axios.post('http://foo.com/login', {}).then(response => console.log(response)); // resolves with a 401 after a 200ms delay

axios
  .get('http://foo.com/some-other-endpoint')
  .then(response => console.log(response)); // resolves with { another: 'response' } after a 1 second delay

In this example, if we load our site up with scenario=failedLogin in the querystring and then attempt to hit the login endpoint, it will fail with a 401. However, the some-other-endpoint endpoint will still respond with the response in the default scenario as we have not provided one in the failedLogin scenario.

Exported interfaces

Scenarios

| Property | Type | Required | Description | | ---------- | ------ | -------- | ------------------------------------------------------------------------------------------------ | | default | Mock[] | ✅ | The default scenario mapping. Provides a default set of mocked responses | | [scenario] | Mock[] | ❌ | Additional scenario mappings. The key is the name of the scenario and is what is used in the URL |

Mock

| Property | Type | Required | Description | | --------------- | ------ | -------- | ------------------------------------------------------------------ | | url | RegExp | ✅ | Regular expression that matches part of the URL | | method | string | ✅ | HTTP method matching one of 'GET', 'POST', 'PUT', 'DELETE' | | response | any | ✅ | Body of the response | | responseCode | number | ❌ | Response code. Defaults to 200 | | responseHeaders | object | ❌ | Response headers. Defaults to empty | | delay | number | ❌ | Delay (in milliseconds) before response is returned. Defaults to 0 |

MockConfig

| Property | Type | Required | Description | | ------------------- | ------- | -------- | --------------------------------------------------------------------------------------------- | | allowXHRPassthrough | boolean | ❌ | Any unmatched routes for XHR will pass through to the actual endpoint, rather than be mocked. |

Exported functions

injectMocks

| Parameter | Type | Required | Description | | --------- | ------------------------------- | -------- | --------------------------------------------------------------------------- | | scenarios | Scenarios[] | ✅ | A mapping of scenarios and their responses | | scenario | keyof Scenarios[] | ❌ | The scenario to run. Defaults to default | | config | MockConfig | ❌ | Config object that allows for different behaviour of how mocks are injected |

extractScenarioFromLocation

| Parameter | Type | Required | Description | | --------- | --------------------------------------------------------------------- | -------- | ---------------------------------------------------------------------------------------------------------------- | | location | Location | ✅ | The browser location object. The value for the scenario part of the querystring will be extracted and returned |