Promise chaining implementation of then()

then-promise.js

Promise.prototype.then = function (onResolve, onReject, onProgress) {
	if(isPromise(onResolve)) {
		// Chain promise
		this.then(
			function(val)    { onResolve.resolve(onReject ? onReject : val); },
			function(err)    { onResolve.reject(err); },
			function(update) { onResolve.progress(update); }
		);
	} else {
		// capture calls to then()
		this._thens.push({ resolve: onResolve, reject: onReject, progress: onProgress });
		onProgress && this._progress.push(onProgress);		
	}

	return this;
};