wormboy / throttleDebounce

/**
 * Debounce execution of a function. Debouncing, unlike throttling,
 * guarantees that a function is only executed a single time, either at the
 * very beginning of a series of calls, or at the very end.
 *
 * @param  {Number}   delay         A zero-or-greater delay in milliseconds. For event callbacks, values around 100 or 250 (or even higher) are most useful.
 * @param  {Boolean}  atBegin       Optional, defaults to false. If atBegin is false or unspecified, callback will only be executed `delay` milliseconds
 *                                  after the last debounced-function call. If atBegin is true, callback will be executed only at the first debounced-function call.
 *                                  (After the throttled-function has not been called for `delay` milliseconds, the internal counter is reset).
 * @param  {Function} callback      A function to be executed after delay milliseconds. The `this` context and all arguments are passed through, as-is,
 *                                  to `callback` when the debounced-function is executed.
 *
 * @return {Function} A new, debounced function.
 */
function throttle(delay, noTrailing, callback, debounceMode) {

	// After wrapper has stopped being called, this timeout ensures that
	// `callback` is executed at the proper times in `throttle` and `end`
	// debounce modes.
	var timeoutID;

	// Keep track of the last time `callback` was executed.
	var lastExec = 0;

	// `noTrailing` defaults to falsy.
	if ( typeof noTrailing !== 'boolean' ) {
		debounceMode = callback;
		callback = noTrailing;
		noTrailing = undefined;
	}

	// The `wrapper` function encapsulates all of the throttling / debouncing
	// functionality and when executed will limit the rate at which `callback`
	// is executed.
	function wrapper () {

		var self = this;
		var elapsed = Number(new Date()) - lastExec;
		var args = arguments;

		// Execute `callback` and update the `lastExec` timestamp.
		function exec () {
			lastExec = Number(new Date());
			callback.apply(self, args);
		}

		// If `debounceMode` is true (at begin) this is used to clear the flag
		// to allow future `callback` executions.
		function clear () {
			timeoutID = undefined;
		}

		if ( debounceMode && !timeoutID ) {
			// Since `wrapper` is being called for the first time and
			// `debounceMode` is true (at begin), execute `callback`.
			exec();
		}

		// Clear any existing timeout.
		if ( timeoutID ) {
			clearTimeout(timeoutID);
		}

		if ( debounceMode === undefined && elapsed > delay ) {
			// In throttle mode, if `delay` time has been exceeded, execute
			// `callback`.
			exec();

		} else if ( noTrailing !== true ) {
			// In trailing throttle mode, since `delay` time has not been
			// exceeded, schedule `callback` to execute `delay` ms after most
			// recent execution.
			//
			// If `debounceMode` is true (at begin), schedule `clear` to execute
			// after `delay` ms.
			//
			// If `debounceMode` is false (at end), schedule `callback` to
			// execute after `delay` ms.
			timeoutID = setTimeout(debounceMode ? clear : exec, debounceMode === undefined ? delay - elapsed : delay);
		}

	}

	// Return the wrapper function.
	return wrapper;
}

function debounce(delay, atBegin, callback) {
	return callback === undefined ? throttle(delay, atBegin, false) : throttle(delay, callback, atBegin !== false);
}