Promise

promise.js

function a (callback) {
	setTimeout(function () {
		console.log('This is a');
		callback && callback(null);
	}, 100);
}

function b (callback) {
	setTimeout(function () {
		console.log('This is b');
		callback && callback();
	}, 200);
}

function c (callback) {
	setTimeout(function () {
		console.log('This is c');
		callback && callback();
	}, 10);
}

function d (callback) {
	setTimeout(function () {
		console.log('This is d');
		callback && callback();
	}, 60);
}

//a(); b(); c(); d():

// a(function () {
// 	b(function () {
// 		c(function () {
// 			d();
// 		});
// 	});
// });

function promise() {
	if (!(this instanceof promise)) {
		var pr = new promise();
		pr.resolve_set = true;
		return pr;
	}
	this.resolve_set = false;
	this.resolve_fn = null;
	this.__check = function () {
		if (this.resolve_set && this.resolve_fn) {
			this.resolve_fn.exec();
		}
	}
}

promise.prototype.resolve = function () {
	this.resolve_set = true;
	this.__check();
}

promise.prototype.then = function (fn) {
	this.resolve_fn = fn.promise();
	this.__check();
	return this.resolve_fn.promise;
}

function promise_obj (fn) {
	this.promise = new promise();
	this.fn = fn;
}

promise_obj.prototype.exec = function () {
	var self = this;
	this.fn(function () {
		self.promise.resolve();
	});
};

function promisify (fn) {
	var ret = function () {
		fn.apply(this, arguments);
	};
	ret.promise = function () {
		return new promise_obj(fn);
	}
	return ret;
}

var _a = promisify(a);
var _b = promisify(b);
var _c = promisify(c);
var _d = promisify(d);

var pr = promise().then(_a).then(_b).then(_c);
_d();
_c();
_c(function () {
	_a();
});
pr.then(_d).then(_a).then(_d);