debounce function

debounce.js

function debounce(func, wait, immediate) {
	var timeout;
	return function() {
		var context = this, args = arguments;
		clearTimeout(timeout);
		timeout = setTimeout(function() {
			timeout = null;
			if (!immediate) func.apply(context, args);
		}, wait);
		if (immediate && !timeout) func.apply(context, args);
	};
};

//example

$('input.username').keypress(debounce(function (event) {
  //do the Ajax request
}, 250));