Promises

promises.js

function MyPromise(action) {
	this.status = 'pending';
	this.value = undefined;

	this.thenCallbacks = [];
	this.onCatch = undefined;
	this.onFinally = undefined;

	this.then = function(callback) {
		this.thenCallbacks.push(callback);
		return this;
	};

	this.catch = function(callback) {
		this.onCatch = callback;
		return this;
	};

	this.finally = function(callback) {
		this.onFinally = callback;
		return this;
	};

	// Do the thing
	action(resolver.bind(this), rejector.bind(this));

	// Private
	function resolver(value) {
		this.status = 'resolved';
		this.value = value;
		this.thenCallbacks.forEach(function(func) {
			func(this.value);
		}, this);

		if (typeof this.finallyCallback === 'function') {
			this.finallyCallback(this.value);
		}
	}

	function rejector(value) {
		this.status = 'rejected';
		this.value = value;

		if (typeof this.catchCallback === 'function') {
			this.catchCallback(this.value);
		}

		if (typeof this.finallyCallback === 'function') {
			this.finallyCallback(this.value);
		}
	}
}

function get(url) {
	return new MyPromise(function(resolve, reject) {
		var request = new XMLHttpRequest();
		request.open('GET', url);

		request.onload = function() {
			if (request.status === 200) {
				resolve(request.response);
			}
			else {
				reject(Error(request.statusText));
			}
		};

		request.onerror = function() {
			reject(Error('Network Error'));
		};

		request.send();
	});
}

get('https://nerboda.github.io')
	.then(function(response) {
		console.log(response);
	})
	.catch(function(error) {
		console.log(error);
	})
	.then(function(result) {
		console.log(result);
	});