throttle.js

/* eslint-disable no-undefined,no-param-reassign,no-shadow */

/**
 * Throttle execution of a function. Especially useful for rate limiting
 * execution of handlers on events like resize and scroll.
 *
 * @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}   noTrailing     Optional, defaults to false. If noTrailing is true, callback will only execute every `delay` milliseconds while the
 *                                    throttled-function is being called. If noTrailing is false or unspecified, callback will be executed one final time
 *                                    after the last throttled-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 throttled-function is executed.
 * @param  {Boolean}   debounceMode   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.
 *
 * @return {Function}  A new, throttled, function.
 */
module.exports = function ( 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;

};