Customizable Promise

Customizable Promise

function Promise(o, fnNames){
	fnNames = fnNames || ['done', 'fail', 'always'];
	var done = fnNames[0],
	    fail = fnNames[1],
	    always = fnNames[2];
		
	o.states = { none: 0 };
	o.states[done] = 1;
	o.states[fail] = 2;
	
	o.handleState = function(){
		if(!this.state) return;		
		var i, l;
		if(this.state === this.states[done]){
			for(i = 0, l = this[done].callbacks.length; i < l ; i++){
				this[done].callbacks[i]();
			}
		} else if(this.state === this.states[fail]){
			for(i = 0, l = this[fail].callbacks.length; i < l ; i++){
				this[fail].callbacks[i]();
			}
		}
		for(i = 0, l = this[always].callbacks.length; i < l ; i++){
			this[always].callbacks[i]();
		}		
	};
	o.state = 0;
	o[done] = function(cb){
		if(this.state === this.states[done]) cb();
		else this[done].callbacks.push(cb);
		return this;
	};
	o[done].callbacks = [];
	o[fail] = function(cb){
		if(this.state === this.states[fail]) cb();
		else this[fail].callbacks.push(cb);
		return this;
	};
	o[fail].callbacks = [];
	o[always] = function(cb){
		if(this.state) cb();
		else this[always].callbacks.push(cb);
		return this;
	};
	o[always].callbacks = [];
		
	return o;
}

Tests

//Normal object wrapper
var someObj = {
		itisdone:function(){
			this.state = this.states.done;
			this.handleState();
		}
	},
	promise = (new Promise(someObj))
		.done(function(){console.log('done')})
		.fail(function(){console.log('fail')})
		.always(function(){console.log('always')})
		.itisdone();
		
//XHR Wrapper
var xhr = new XMLHttpRequest(),
	request = new Promise(xhr, ['success', 'error', 'finally']);

request.onload = function(){
	this.state = this.status >= 200 && this.status < 400 ? this.states.done : this.states.fail;
	this.handleState();
};

request
	.success(function(){console.log('success')})
	.error(function(){console.log('error')})
	.finally(function(){console.log('finally')});