Pretender

Pretender is a mock server library in the style of Sinon (but built from microlibs. Because JavaScript) that comes with an express/sinatra style syntax for defining routes and their handlers.

Pretender will temporarily replace the native XMLHttpRequest object, intercept all requests, and direct them to little pretend service you've defined.

⚠️ Pretender only works in the browser!

var PHOTOS = {
  "10": {
    id: 10,
    src: 'http://media.giphy.com/media/UdqUo8xvEcvgA/giphy.gif'
  },
  "42": {
    id: 42,
    src: 'http://media0.giphy.com/media/Ko2pyD26RdYRi/giphy.gif'
  }
};

var server = new Pretender(function(){
  this.get('/photos', function(request){
    var all =  JSON.stringify(Object.keys(PHOTOS).map(function(k){return PHOTOS[k]}))
    return [200, {"Content-Type": "application/json"}, all]
  });

  this.get('/photos/:id', function(request){
    return [200, {"Content-Type": "application/json"}, JSON.stringify(PHOTOS[request.params.id])]
  });
});

$.get('/photos/12', {success: function(){ ... }})

The Server DSL

The server DSL is inspired by express/sinatra. Pass a function to the Pretender constructor that will be invoked with the Pretender instance as its context. Available methods are get, put, post, 'delete', patch, and head. Each of these methods takes a path pattern, a callback, and an optional timing parameter. The callback will be invoked with a single argument (the XMLHttpRequest instance that triggered this request) and must return an array containing the HTTP status code, headers object, and body as a string.

var server = new Pretender(function(){
  this.put('/api/songs/99', function(request){
    return [404, {}, ""];
  });
});

a Pretender constructor can take multiple maps:

import adminMaps from "testing/maps/admin";
import photoMaps from "testing/maps/photos";

var server = new Pretender(photoMaps, adminMaps);

The HTTP verb methods can also be called on an instance individually:

var server = new Pretender();
server.put('/api/songs/99', function(request){
  return [404, {}, ""];
});

Paths

Paths can either be hard-coded (this.get('/api/songs/12')) or contain dynamic segments (this.get('/api/songs/:song_id'). If there were dynamic segments of the path, these well be attached to the request object as a params property with keys matching the dynamic portion and values with the matching value from the path.

var server = new Pretender(function(){
  this.get('/api/songs/:song_id', function(request){
    request.params.song_id;
  });
});

$.get('/api/songs/871') // params.song_id will be '871'

Query Parameters

If there were query parameters in the request, these well be attached to the request object as a queryParams property.

var server = new Pretender(function(){
  this.get('/api/songs', function(request){
    request.queryParams.sortOrder;
  });
});

// typical jQuery-style uses you've probably seen.
// queryParams.sortOrder will be 'asc' for both styles.
$.get({url: '/api/songs', data: {sortOrder: 'asc'});
$.get('/api/songs?sortOrder=asc');

Responding

You must return an array from this handler that includes the HTTP status code, an object literal of response headers, and a string body.

var server = new Pretender(function(){
  this.get('/api/songs', function(request){
    return [
      200,
      {'content-type': 'application/javascript'},
      '[{"id": 12}, {"id": 14}]'
    ];
  });
});

Or, optionally, return a Promise.

var server = new Pretender(function() {
  this.get('/api/songs', function(request) {
    return new Promise(function(resolve) {
      var response = [
        200,
        {'content-type': 'application/javascript'},
        '[{"id": 12}, {"id": 14}]'
      ];
      resolve(response);
    });
  });
});

Pass-Through

You can specify paths that should be ignored by pretender and made as real XHR requests. Enable these by specifying pass-through routes with pretender.passthrough:

var server = new Pretender(function(){
  this.get('/photos/:id', this.passthrough);
});

In some case, you will need to force pretender to passthough, just start your server with the forcePassthrough option.

var server = new Pretender({ forcePassthrough: true })

Timing Parameter

The timing parameter is used to control when a request responds. By default, a request responds asynchronously on the next frame of the browser's event loop. A request can also be configured to respond synchronously, after a defined amount of time, or never (i.e., it needs to be manually resolved).

Default

var server = new Pretender(function(){
  // songHandler will execute the frame after receiving a request (async)
  this.get('/api/songs', songHandler);
});

Synchronous

var server = new Pretender(function(){
  // songHandler will execute immediately after receiving a request (sync)
  this.get('/api/songs', songHandler, false);
});

Delay

var server = new Pretender(function(){
  // songHandler will execute two seconds after receiving a request (async)
  this.get('/api/songs', songHandler, 2000);
});

Manual

var server = new Pretender(function(){
  // songHandler will only execute once you manually resolve the request
  this.get('/api/songs', songHandler, true);
});

// resolve a request like this
server.resolve(theXMLHttpRequestThatRequestedTheSongsRoute);

Using functions for the timing parameter

You may want the timing behavior of a response to change from request to request. This can be done by providing a function as the timing parameter.

var externalState = 'idle';

function throttler() {
  if (externalState === 'OH NO DDOS ATTACK') {
    return 15000;
  }
}

var server = new Pretender(function(){
  // songHandler will only execute based on the result of throttler
  this.get('/api/songs', songHandler, throttler);
});

Now whenever the songs route is requested, its timing behavior will be determined by the result of the call to throttler. When externalState is idle, throttler returns undefined, which means the route will use the default behavior.

When the time is right, you can set externalState to "OH NO DOS ATTACK" which will make all future requests take 15 seconds to respond.

Sharing routes

You can call map multiple times on a Pretender instance. This is a great way to share and reuse sets of routes between tests:

export function authenticationRoutes(){
  this.post('/authenticate', function(){ ... });
  this.post('/signout', function(){ ... });
}

export function songsRoutes(){
  this.get('/api/songs', function(){ ... });
}

// a test

import {authenticationRoutes, songsRoutes} from "../shared/routes";
import Pretender from "pretender";

let p = new Pretender();
p.map(authenticationRoutes);
p.map(songsRoutes);

Hooks

Handled Requests

In addition to responding to the request, your server will call a handledRequest method with the HTTP verb, path, and original request. By default this method does nothing. You can override this method to supply your own behavior like logging or test framework integration:

var server = new Pretender(function(){
  this.put('/api/songs/:song_id', function(request){
    return [202, {"Content-Type": "application/json"}, "{}"]
  });
});

server.handledRequest = function(verb, path, request) {
  console.log("a request was responded to");
}

$.getJSON("/api/songs/12");

Unhandled Requests

Your server will call a unhandledRequest method with the HTTP verb, path, and original request, object if your server receives a request for a route that doesn't have a handler. By default, this method will throw an error. You can override this method to supply your own behavior:

var server = new Pretender(function(){
  // no routes
});

server.unhandledRequest = function(verb, path, request) {
  console.log("what is this I don't even...");
}

$.getJSON("/these/arent/the/droids");

Pass-through Requests

Requests set to be handled by pass-through will trigger the passthroughRequest hook:

var server = new Pretender(function(){
  this.get('/some/path', this.passthrough);
});

server.passthroughRequest = function(verb, path, request) {
  console.log('request ' + path + ' sucessfully sent for passthrough');
}

Error Requests

Your server will call a erroredRequest method with the HTTP verb, path, original request, and the original error object if your handler code causes an error.

By default, this will augment the error message with some information about which handler caused the error and then throw the error again. You can override this method to supply your own behavior:

var server = new Pretender(function(){
  this.get('/api/songs', function(request){
    undefinedWAT("this is no function!");
  });
});

server.erroredRequest = function(verb, path, request, error) {
  SomeTestFramework.failTest();
  console.warn("There was an error", error);
}

Mutating the body

Pretender is response format neutral, so you normally need to supply a string body as the third part of a response:

this.get('/api/songs', function(request){
  return [200, {}, "{'id': 12}"];
});

This can become tiresome if you know, for example, that all your responses are going to be JSON. The body of a response will be passed through a prepareBody hook before being passed to the fake response object. prepareBody defaults to an empty function, but can be overridden:

var server = new Pretender(function(){
  this.get('/api/songs', function(request){
    return [200, {}, {id: 12}];
  });
});

server.prepareBody = function(body){
  return body ? JSON.stringify(body) : '{"error": "not found"}';
}

Mutating the headers

Response headers can be mutated for the entire service instance by implementing a prepareHeaders method:

var server = new Pretender(function(){
  this.get('/api/songs', function(request){
    return [200, {}, '{"id": 12}'];
  });
});

server.prepareHeaders = function(headers){
  headers['content-type'] = 'application/javascript';
  return headers;
};

Tracking Requests

Your pretender instance will track handlers and requests on a few array properties. All handlers are stored on handlers property and incoming requests will be tracked in one of three properties: handledRequests, unhandledRequests and passthroughRequests. The handler is also returned from any verb function. This is useful if you want to build testing infrastructure on top of pretender and need to fail tests that have handlers without requests. You can disable tracking requests by passing trackRequests: false to pretender options.

var server = new Pretender({ trackRequests: false });

Each handler keeps a count of the number of requests is successfully served.

server.get(/* ... */);
var handler = server.handlers[0];

// or

var handler = server.get(/* ... */);

// then

var numberOfCalls = handler.numberOfCalls;

Clean up

When you're done mocking, be sure to call shutdown() to restore the native XMLHttpRequest object:

var server = new Pretender(function(){
 ... routing ...
});

server.shutdown(); // all done.

Development of Pretender

Running tests

• npm test runs tests once
• npm run test:server runs and reruns on changes

Code of Conduct

In order to have a more open and welcoming community this project adheres to a code of conduct adapted from the contributor covenant.

Please adhere to this code of conduct in any interactions you have with this project's community. If you encounter someone violating these terms, please let a maintainer (@trek) know and we will address it as soon as possible.