throttle.js
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/* 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;
};