/* eslint-disable no-undefined,no-param-reassign,no-shadow */
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/**
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* Throttle execution of a function. Especially useful for rate limiting
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* execution of handlers on events like resize and scroll.
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*
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* @param {Number} delay A zero-or-greater delay in milliseconds. For event callbacks, values around 100 or 250 (or even higher) are most useful.
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* @param {Boolean} [noTrailing] Optional, defaults to false. If noTrailing is true, callback will only execute every `delay` milliseconds while the
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* throttled-function is being called. If noTrailing is false or unspecified, callback will be executed one final time
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* after the last throttled-function call. (After the throttled-function has not been called for `delay` milliseconds,
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* the internal counter is reset)
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* @param {Function} callback A function to be executed after delay milliseconds. The `this` context and all arguments are passed through, as-is,
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* to `callback` when the throttled-function is executed.
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* @param {Boolean} [debounceMode] If `debounceMode` is true (at begin), schedule `clear` to execute after `delay` ms. If `debounceMode` is false (at end),
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* schedule `callback` to execute after `delay` ms.
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*
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* @return {Function} A new, throttled, function.
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*/
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module.exports = function ( delay, noTrailing, callback, debounceMode ) {
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// After wrapper has stopped being called, this timeout ensures that
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// `callback` is executed at the proper times in `throttle` and `end`
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// debounce modes.
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var timeoutID;
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// Keep track of the last time `callback` was executed.
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var lastExec = 0;
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// `noTrailing` defaults to falsy.
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if ( typeof noTrailing !== 'boolean' ) {
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debounceMode = callback;
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callback = noTrailing;
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noTrailing = undefined;
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}
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// The `wrapper` function encapsulates all of the throttling / debouncing
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// functionality and when executed will limit the rate at which `callback`
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// is executed.
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function wrapper () {
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var self = this;
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var elapsed = Number(new Date()) - lastExec;
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var args = arguments;
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// Execute `callback` and update the `lastExec` timestamp.
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function exec () {
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lastExec = Number(new Date());
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callback.apply(self, args);
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}
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// If `debounceMode` is true (at begin) this is used to clear the flag
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// to allow future `callback` executions.
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function clear () {
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timeoutID = undefined;
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}
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if ( debounceMode && !timeoutID ) {
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// Since `wrapper` is being called for the first time and
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// `debounceMode` is true (at begin), execute `callback`.
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exec();
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}
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// Clear any existing timeout.
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if ( timeoutID ) {
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clearTimeout(timeoutID);
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}
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if ( debounceMode === undefined && elapsed > delay ) {
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// In throttle mode, if `delay` time has been exceeded, execute
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// `callback`.
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exec();
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} else if ( noTrailing !== true ) {
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// In trailing throttle mode, since `delay` time has not been
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// exceeded, schedule `callback` to execute `delay` ms after most
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// recent execution.
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//
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// If `debounceMode` is true (at begin), schedule `clear` to execute
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// after `delay` ms.
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//
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// If `debounceMode` is false (at end), schedule `callback` to
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// execute after `delay` ms.
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timeoutID = setTimeout(debounceMode ? clear : exec, debounceMode === undefined ? delay - elapsed : delay);
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}
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}
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// Return the wrapper function.
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return wrapper;
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};
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