How do I promisify native XHR?

Using callbacks

I want to use (native) promises in my frontend app to perform XHR request but without all the tomfoolery of a massive framework.
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function makeRequest (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:
makeRequest('GET', 'http://example.com', function (err, datums) {
if (err) { throw err; }
console.log(datums);
});

How do I promisify native XHR?

The promise constructor

We can construct a promise like so:

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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 makeRequest with this constructor:

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function makeRequest (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:
makeRequest('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):

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makeRequest('GET', 'http://example.com')
.then(function (datums) {
return makeRequest('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:

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

makeRequest now looks something like this:

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function makeRequest (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
makeRequest({
method: 'GET',
url: 'http://example.com'
})
.then(function (datums) {
return makeRequest({
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);
});