Ajax library with XHR2, promises and request limit

  • qwest

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4.5.07 years ago10 years agoMinified + gzip package size for qwest in KB


qwest 4.5.0
A complete rewrite of qwest in ES6 with many improvements is planned soon. Keep in touch!
Qwest is a simple ajax library based on promises and that supports XmlHttpRequest2 special data like ArrayBuffer, Blob and FormData.


npm install qwest
bower install qwest

Qwest is also available via CDNJS : https://cdnjs.com/libraries/qwest
If you need to import qwest in TypeScript, do :
import * as qwest from 'qwest';

Quick examples

     .then(function(xhr, response) {

qwest.post('example.com', {
        firstname: 'Pedro',
        lastname: 'Sanchez',
        age: 30
     .then(function(xhr, response) {
        // Make some useful actions
     .catch(function(e, xhr, response) {
        // Process the error


qwest.`method`(`url`, `data`, `options`, `before`)
     .then(function(xhr, response) {
        // Run when the request is successful
     .catch(function(e, xhr, response) {
        // Process the error
     .complete(function() {
         // Always run

The method is either get, post, put or delete. The data parameter can be a multi-dimensional array or object, a string, an ArrayBuffer, a Blob, etc... If you don't want to pass any data but specify some options, set data to null.
The available options are :
  • dataType : post (by default), queryString, json, text, arraybuffer, blob, document or formdata (you shouldn't need to specify XHR2 types since they're automatically detected)
  • responseType : the response type; either auto (default), json, xml, text, arraybuffer, blob or document
  • cache : browser caching; default is false
  • async : true (default) or false; used to make asynchronous or synchronous requests
  • user : the user to access to the URL, if needed
  • password : the password to access to the URL, if needed
  • headers : javascript object containing headers to be sent
  • withCredentials : false by default; sends credentials with your XHR2 request (more info in that post)
  • timeout : the timeout for the request in ms; 30000 by default (allowed only in async mode)
  • attempts : the total number of times to attempt the request through timeouts; 1 by default; if you want to remove the limit set it to null

You can change the default data type with :

If you want to make a call with another HTTP method, you can use the map() function :
qwest.map('PATCH', 'example.com')
     .then(function() {
         // Blah blah

If you need to do a sync request, you must call send() at the end of your promise :
qwest.get('example.com', {async: false})
     .then(function() {
         // Blah blah

Since service APIs often need the same type of request, you can set default options for all of your requests with :
    dataType: 'arraybuffer',
    responseType: 'json',
    headers: {
        'My-Header': 'Some-Value'

Note : if you want to send your data as a query string parameter chain, pass queryString to the dataType option.

Group requests

Sometimes we need to call several requests and execute some tasks after all of them are completed. You can simply do it by chaining your requests like :
     .post('example.com/login', auth_data)
     .then(function(values) {
            Prints [ [xhr, response], [xhr, response], [xhr, response] ]

If an error is encountered then catch() will be called and all requests will be aborted.

Base URI

You can define a base URI for your requests. The string will be prepended to the other request URIs.
qwest.base = 'http://example.com';

// Will make a request to 'http://example.com/somepage'
     .then(function() {
         // Blah blah

Request limit

One of the greatest qwest functionnalities is the request limit. It avoids browser freezes and server overloads by freeing bandwidth and memory resources when you have a whole bunch of requests to do at the same time. Set the request limit and when the count is reached qwest will stock all further requests and start them when a slot is free.
Let's say we have a gallery with a lot of images to load. We don't want the browser to download all images at the same time to have a faster loading. Let's see how we can do that.
<div class="gallery">
    <img data-src="images/image1.jpg" alt="">
    <img data-src="images/image2.jpg" alt="">
    <img data-src="images/image3.jpg" alt="">
    <img data-src="images/image4.jpg" alt="">
    <img data-src="images/image5.jpg" alt="">

// Browsers are limited in number of parallel downloads, setting it to 4 seems fair

$('.gallery').children().forEach(function() {
    var $this = $(this);
    qwest.get($this.data('src'), {responseType: 'blob'})
         .then(function(xhr, response) {
            $this.attr('src', window.URL.createObjectURL(response));

If you want to remove the limit, set it to null.

CORS and preflight requests

According to #90 and #99, a CORS request will send a preflight OPTIONS request to the server to know what is allowed and what's not. It's because we're adding a Cache-Control header to handle caching of requests. The simplest way to avoid this OPTIONS request is to set cache option to true. If you want to know more about preflight requests and how to really handle them, read this : https://developer.mozilla.org/en-US/docs/Web/HTTP/AccesscontrolCORS

Aborting a request

// Start the request
var request = qwest.get('example.com')
                   .then(function(xhr, response) {
                       // Won't be called
                   .catch(function(xhr, response) {
                       // Won't be called either

// Some code


Not that only works with asynchroneous requests since synchroneous requests are... synchroneous.

Set options to the XHR object

If you want to apply some manual options to the XHR object, you can use the before option
qwest.get('example.com', null, null, function(xhr) {
        xhr.upload.onprogress = function(e) {
            // Upload in progress
     .then(function(xhr, response) {
        // Blah blah blah

Handling fallbacks

XHR2 is not available on every browser, so, if needed, you can simply verify the XHR version with :
if(qwest.xhr2) {
    // Actions for XHR2
else {
    // Actions for XHR1

Receiving binary data in older browsers

Getting binary data in legacy browsers needs a trick, as we can read it on MDN. In qwest, that's how we could handle it :
qwest.get('example.com/file', null, null, function(xhr) {
        xhr.overrideMimeType('text\/plain; charset=x-user-defined');
     .then(function(response) {
         // response is now a binary string

Compatibility notes

According to this compatibility table, IE7/8 do not support using catch and delete as method name because these are reserved words. If you want to support those browsers you should write :

Like this :

XHR2 does not support arraybuffer, blob and document response types in synchroneous mode.
The CORS object shipped with IE8 and 9 is XDomainRequest. This object does not support PUT and DELETE requests and XHR2 types. Moreover, the getResponseHeader() method is not supported too which is used in the auto mode for detecting the reponse type. Then, the response type automatically fallbacks to json when in auto mode. If you expect another response type, please specify it explicitly. If you want to specify another default response type to fallback in auto mode, you can do it like this :

Last notes

  • Blackberry 10.2.0 (and maybe others) can log an error saying json is not supported : set responseType to auto to avoid the issue
  • the catch handler will be executed for status codes different from 2xx; if no data has been received when catch is called, response will be null
  • auto mode is only supported for xml, json and text response types; for arraybuffer, blob and document you'll need to define explicitly the responseType option
  • if the response of your request doesn't return a valid (and recognized) Content-Type header, then you must explicitly set the responseType option
  • the default Content-Type header for a POST request is application/x-www-form-urlencoded, for post and xhr2 data types
  • if you want to set or get raw data, set dataType option to text
  • as stated on StackOverflow, XDomainRequest forbid HTTPS requests from HTTP scheme and vice versa
  • XDomainRequest only supports GET and POST methods


MIT license everywhere!