gistfile1.js

function makeProgressPromise(cb) {

	var currentProgress = 0;
	var progressListeners = [];

	function updateProgress(newProgress) {
		currentProgress = newProgress;
		progressListeners.forEach(function(listener) {
			try {
				listener(currentProgress);	
			} catch (e) {
				// swallow errors in listeners
			}
			
		});
	}

	var p = new Promise(function(yes, no) {
		cb(yes, no, updateProgress);
	});

	p.progress = function(cb) {
		progressListeners.push(cb);
		// update new listeners with current progress immediately
		cb(currentProgress);
		return this;
	}

	return p;

}

function ajax(method, url) {
	return makeProgressPromise(function(yes, no, progress) {
		// perform operation, call progress() with updates
	});
}

//
// Usage

ajax('GET', '/orders.php')
	.progress(function(p) { /* update progress meter with value of `p` */ })
	.then(function(result) {
		// ...
	})