promisify XHR

canon.md

How do I promisify native XHR?

I want to use (native) promises in my frontend app to perform XHR request but without all the tomfoolery of a massive framework.

I want my xhr to return a promise but this doesn't work (giving me: Uncaught TypeError: Promise resolver undefined is not a function)

function makeXHRRequest (method, url, done) {
  var xhr = new XMLHttpRequest();
  xhr.open(method, url);
  xhr.onload = function() { return new Promise().resolve(); };
  xhr.onerror = function() { return new Promise().reject(); };
  xhr.send();
}

I'm assuming you know how to make a native XHR request (you can brush up here and here)

Since any browser that supports native promises will also support xhr.onload, we can skip all the onReadyStateChange tomfoolery. Let's take a step back and start with a basic XHR request function using callbacks:

function makeXHRRequest (method, url, done) {
  var xhr = new XMLHttpRequest();
  xhr.open(method, url);
  xhr.onload = function () {
    done(null, xhr.response);
  };
  xhr.onerror = function () {
    done(xhr.response);
  };
  xhr.send();
}

// And we'd call it as such:

makeXHRRequest('GET', 'http://example.com', function (err, datums) {
  if (err) { throw err; }
  console.log(datums);
});

Hurrah! This doesn't involve anything terribly complicated (like custom headers or POST data) but is enough to get us moving forwards.

##The promise constructor

We can construct a promise like so:

new Promise(function (resolve, reject) {
  // Do some Async stuff
  // call resolve if it succeeded
  // reject if it failed
});

The promise constructor takes a function that will be passed two arguments (let's call them resolve and reject). You can think of these as callbacks, one for success and one for failure. Examples are awesome, let's update makeXHRRequest with this constructor:

function makeXHRRequest (method, url) {
  return new Promise(function (resolve, reject) {
    var xhr = new XMLHttpRequest();
    xhr.open(method, url);
    xhr.onload = function () {
      if (this.status >= 200 && this.status < 300) {
        resolve(xhr.response);
      } else {
        reject({
          status: this.status,
          statusText: xhr.statusText
        });
      }
    };
    xhr.onerror = function () {
      reject({
        status: this.status,
        statusText: xhr.statusText
      });
    };
    xhr.send();
  });
}

// Example:

makeXHRRequest('GET', 'http://example.com')
.then(function (datums) {
  console.log(datums);
})
.catch(function (err) {
  console.error('Augh, there was an error!', err.statusText);
});

Now we can tap into the power of promises, chaining multiple XHR calls (and the .catch will trigger for an error on either call):

makeXHRRequest('GET', 'http://example.com')
.then(function (datums) {
  return makeXHRRequest('GET', datums.url);
})
.then(function (moreDatums) {
  console.log(moreDatums);
})
.catch(function (err) {
  console.error('Augh, there was an error!', err.statusText);
});

We can improve this still further, adding both POST/PUT params and custom headers. Let's use an options object instead of multiple arguments, with the signature:

{
  method: String,
  url: String,
  params: String | Object,
  headers: Object
}

makeXHRRequest now looks something like this:

function makeXHRRequest (opts) {
  return new Promise(function (resolve, reject) {
    var xhr = new XMLHttpRequest();
    xhr.open(opts.method, opts.url);
    xhr.onload = function () {
      if (this.status >= 200 && this.status < 300) {
        resolve(xhr.response);
      } else {
        reject({
          status: this.status,
          statusText: xhr.statusText
        });
      }
    };
    xhr.onerror = function () {
      reject({
        status: this.status,
        statusText: xhr.statusText
      });
    };
    if (opts.headers) {
      Object.keys(opts.headers).forEach(function (key) {
        xhr.setRequestHeader(key, opts.headers[key]);
      });
    }
    var params = opts.params;
    // We'll need to stringify if we've been given an object
    // If we have a string, this is skipped.
    if (params && typeof params === 'object') {
      params = Object.keys(params).map(function (key) {
        return encodeURIComponent(key) + '=' + encodeURIComponent(params[key]);
      }).join('&');
    }
    xhr.send(params);
  });
}

// Headers and params are optional
makeXHRRequest({
  method: 'GET',
  url: 'http://example.com'
})
.then(function (datums) {
  return makeXHRRequest({
    method: 'POST',
    url: datums.url,
    params: {
      score: 9001
    },
    headers: {
      'X-Subliminal-Message': 'Upvote-this-answer'
    }
  });
})
.catch(function (err) {
  console.error('Augh, there was an error!', err.statusText);
});

A more comprehensive approach can be found at MDN.

Alternatively, you could use the fetch API (polyfill) but this is not likely to become standard, as there is some confusion around cancellation.