Pretender

Pretender is a mock server library for XMLHttpRequest and Fetch, that comes with an express/sinatra style syntax for defining routes and their handlers.

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

⚠️ Pretender only works in the browser!

constPHOTOS={"10": {id: 10,src: 'http://media.giphy.com/media/UdqUo8xvEcvgA/giphy.gif'},"42": {id: 42,src: 'http://media0.giphy.com/media/Ko2pyD26RdYRi/giphy.gif'}};constserver=newPretender(function(){this.get('/photos',request=>{letall=JSON.stringify(Object.keys(PHOTOS).map(k=>PHOTOS[k]));return[200,{"Content-Type": "application/json"},all]});this.get('/photos/:id',request=>{return[200,{"Content-Type": "application/json"},JSON.stringify(PHOTOS[request.params.id])]});});$.get('/photos/12',{success()=>{ ... }})

Usage

yarn add -D pretender
# or
npm install --save-dev pretender

You can load Pretender directly in the browser.

<scriptsrc="node_modules/pretender/dist/pretender.bundle.js"></script>

Or as a module:

importPretenderfrom'pretender';constserver=newPretender(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.

constserver=newPretender(function(){this.put('/api/songs/99',request=>[404,{},""]);});

a Pretender constructor can take multiple maps:

importadminMapsfrom"testing/maps/admin";importphotoMapsfrom"testing/maps/photos";constserver=newPretender(photoMaps,adminMaps);

// testing/maps/photosconstPHOTOS={"58": {id: 58,src: 'https://media.giphy.com/media/65TpAhHZ7A9nuf3GIu/giphy.gif'},"99": {id: 99,src: 'https://media.giphy.com/media/4Zd5qAcFv759xnegdo/giphy.gif'}};exportdefaultfunction(){this.get('/photos/:id',()=>[200,{"Content-Type": "application/json"},JSON.stringify(PHOTOS[request.params.id])]);}

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

constserver=newPretender();server.put('/api/songs/99',request=>[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 will 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.

constserver=newPretender(function(){this.get('/api/songs/:song_id',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 will be attached to the request object as a queryParams property.

constserver=newPretender(function(){this.get('/api/songs',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.

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

Or, optionally, return a Promise.

constserver=newPretender(function(){this.get('/api/songs',request=>{returnnewPromise(resolve=>{letresponse=[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:

constserver=newPretender(function(){this.get('/photos/:id',this.passthrough);});

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

constserver=newPretender({forcePassthrough: true})

Other times, you may want to decide whether or not to passthrough when the call is made. In that case you can use the .passthrough() function on the fake request itself. (The unhandledRequest property is discussed below.)

server.unhandledRequest=function(verb,path,request){if(myIgnoreRequestChecker(path)){console.warn(`Ignoring request) ${verb.toUpperCase()} : ${path}`);}else{console.warn(`Unhandled ${verb.toUpperCase()} : ${path} >> Passing along. See eventual response below.`)constxhr=request.passthrough();// <-- A native, sent xhr is returnedxhr.onloadend=(ev)=>{console.warn(`Response for ${path}`,{
verb,
path,
request,responseEvent: ev,})};}};

The .passthrough() function will immediately create, send, and return a native XMLHttpRequest.

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

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

Synchronous

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

Delay

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

Manual

constserver=newPretender(function(){// songHandler will only execute once you manually resolve the requestthis.get('/api/songs',songHandler,true);});// resolve a request like thisserver.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.

constexternalState='idle';functionthrottler(){if(externalState==='OH NO DDOS ATTACK'){return15000;}}constserver=newPretender(function(){// songHandler will only execute based on the result of throttlerthis.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.

Scheduling ProgressEvent

If the timing parameter is resolved as async, then a ProgressEvent will be scheduled every 50ms until the request has a response or is aborted.

To listen to the progress, you can define onprogress on the XMLHttpRequest object or its upload attribute.

letxhr=newwindow.XMLHttpRequest();xhr.open('POST','/uploads');// https://fetch.spec.whatwg.org/#concept-request-body// https://xhr.spec.whatwg.org/#the-send()-methodletpostBody=newArrayBuffer(8);xhr.upload.onprogress=function(event){// event.lengthComputable === true// event.total === 8// event.loaded will be incremented every ~50ms};xhr.onprogress=function(event){// xhr onprogress will also be triggered};xhr.send(postBody);

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:

exportfunctionauthenticationRoutes(){this.post('/authenticate',()=>{ ... });this.post('/signout',()=>{ ... });}exportfunctionsongsRoutes(){this.get('/api/songs',()=>{ ... });}

// a testimport{authenticationRoutes,songsRoutes}from"../shared/routes";importPretenderfrom"pretender";letp=newPretender();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:

constserver=newPretender(function(){this.put('/api/songs/:song_id',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:

constserver=newPretender(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:

constserver=newPretender(function(){this.get('/some/path',this.passthrough);});server.passthroughRequest=function(verb,path,request){console.log('request '+path+' successfully 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:

constserver=newPretender(function(){this.get('/api/songs',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',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:

constserver=newPretender(function(){this.get('/api/songs',request=>{return[200,{},{id: 12}];});});server.prepareBody=function(body){returnbody ? JSON.stringify(body) : '{"error": "not found"}';}

Mutating the headers

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

constserver=newPretender(function(){this.get('/api/songs',request=>{return[200,{},'{"id": 12}'];});});server.prepareHeaders=function(headers){headers['content-type']='application/javascript';returnheaders;};

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.

constserver=newPretender({trackRequests: false});

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

server.get(/* ... */);consthandler=server.handlers[0];// orconsthandler=server.get(/* ... */);// thenconstnumberOfCalls=handler.numberOfCalls;

Clean up

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

constserver=newPretender(function(){
... routing...});server.shutdown();// all done.

Development of Pretender

Running tests

• yarn build builds pretender
• yarn test runs tests once
• yarn 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.