浅谈JavaScript节流和防抖函数

概念

节流函数

间隔固定的时间执行传入的方法

目的是防止函数执行的频率过快,影响性能.常见于跟滚动,鼠标移动事件绑定的功能.

防抖函数

对于接触过硬件的人也许更好理解,硬件按钮按下时,由于用户按住时间的长短不一,会多次触发电流的波动,加一个防抖函数就会只触发一次,防止了无意义的电流波动引起的问题.

按键防反跳（Debounce）为什么要去抖动呢？机械按键在按下时，并非按下就接触的很好，尤其是有簧片的机械开关，会在接触的瞬间反复的开合多次，直到开关状态完全改变。

应用在前端时,常见的场景是,输入框打字动作结束一段时间后再去触发查询/搜索/校验,而不是每打一个字都要去触发,造成无意义的ajax查询等,或者与调整窗口大小绑定的函数,其实只需要在最后窗口大小固定之后再去执行动作.

自己的实现

防抖函数

关键点在于每次触发时都清空延时函数的手柄,只有最后一次触发不会清空手柄,所以最后一次触发会等默认的1s后去执行debounce传入的参数函数f. debounce内部返回的闭包函数,是真正每次被调用触发的函数,不再是原本的f,所以这里的arguments取闭包函数环境变量中的arguments并在执行f时传给f,在setTimeout函数的外面取得.

let debounce = function(f, interval = 1000) { let handler = null; return function() { if (handler) { clearTimeout(handler); } let arg = arguments; handler = setTimeout(function() { f.apply(this, arg); clearTimeout(handler); }, interval) } }

应用:

let input = document.querySelector(’#input’); input.addEventListener(’input’, debounce(function(e) { console.log('您的输入是',e.target.value) }))

更高级的实现还会考虑到,以leading和trailing作为参数,起始先执行一次函数并消除后面的抖动,还是最后执行一下函数,消除前面的抖动,如同我这里的例子.后面分析loadash的防抖函数时会详细解析.

节流函数

let throttle = function(f,gap = 300){ let lastCall = 0; return function(){ let now = Date.now(); let ellapsed = now - lastCall; if(ellapsed < gap){ return } f.apply(this,arguments); lastCall = Date.now(); } }

闭包函数在不断被调用的期间,去记录离上一次调用间隔的时间,如果间隔时间小于节流设置的时间则直接返回,不去执行真正被包裹的函数f.只有间隔时间大于了节流函数设置的时间gap,才调用f,并更新调用时间.

应用:

document.addEventListener(’scroll’, throttle(function (e) { // 判断是否滚动到底部的逻辑 console.log(e,document.documentElement.scrollTop); }));

lodash源码分析

以上是对节流防抖函数最基础简单的实现,我们接下来分析一下lodash库中节流防抖函数的分析.

节流函数的使用

$(window).on(’scroll’, _.debounce(doSomething, 200));

function debounce(func, wait, options) { var lastArgs, lastThis, result, timerId, lastCallTime = 0, lastInvokeTime = 0, leading = false, maxWait = false, trailing = true; if (typeof func != ’function’) { throw new TypeError(FUNC_ERROR_TEXT); } wait = wait || 0; if (isObject(options)) { leading = !!options.leading; maxWait = ’maxWait’ in options && Math.max((options.maxWait) || 0, wait); trailing = ’trailing’ in options ? !!options.trailing : trailing; } function invokeFunc(time) { var args = lastArgs, thisArg = lastThis; lastArgs = lastThis = undefined; lastInvokeTime = time; result = func.apply(thisArg, args); return result; } function leadingEdge(time) { console.log('leadingEdge setTimeout') // Reset any `maxWait` timer. lastInvokeTime = time; // Start the timer for the trailing edge. timerId = setTimeout(timerExpired, wait); // Invoke the leading edge. return leading ? invokeFunc(time) : result; } function remainingWait(time) { var timeSinceLastCall = time - lastCallTime, timeSinceLastInvoke = time - lastInvokeTime, result = wait - timeSinceLastCall; console.log('remainingWait',result) return maxWait === false ? result : Math.min(result, maxWait - timeSinceLastInvoke); } function shouldInvoke(time) { console.log('shouldInvoke') var timeSinceLastCall = time - lastCallTime, timeSinceLastInvoke = time - lastInvokeTime; console.log('time',time,'lastCallTime',lastCallTime,'timeSinceLastCall',timeSinceLastCall) console.log('time',time,'lastInvokeTime',lastInvokeTime,'timeSinceLastInvoke',timeSinceLastInvoke) console.log('should?',(!lastCallTime || (timeSinceLastCall >= wait) || (timeSinceLastCall < 0) || (maxWait !== false && timeSinceLastInvoke >= maxWait))) // Either this is the first call, activity has stopped and we’re at the // trailing edge, the system time has gone backwards and we’re treating // it as the trailing edge, or we’ve hit the `maxWait` limit. return (!lastCallTime || (timeSinceLastCall >= wait) || (timeSinceLastCall < 0) || (maxWait !== false && timeSinceLastInvoke >= maxWait)); } function timerExpired() { console.log('timerExpired') var time = Date.now(); if (shouldInvoke(time)) { return trailingEdge(time); } console.log('Restart the timer.',time,remainingWait(time)) // Restart the timer. console.log('timerExpired setTimeout') timerId = setTimeout(timerExpired, remainingWait(time)); } function trailingEdge(time) { clearTimeout(timerId); timerId = undefined; // Only invoke if we have `lastArgs` which means `func` has been // debounced at least once. console.log('trailing',trailing,'lastArgs',lastArgs) if (trailing && lastArgs) { return invokeFunc(time); } lastArgs = lastThis = undefined; return result; } function cancel() { if (timerId !== undefined) { clearTimeout(timerId); } lastCallTime = lastInvokeTime = 0; lastArgs = lastThis = timerId = undefined; } function flush() { return timerId === undefined ? result : trailingEdge(Date.now()); } function debounced() { var time = Date.now(), isInvoking = shouldInvoke(time); console.log('time',time); console.log('isInvoking',isInvoking); lastArgs = arguments; lastThis = this; lastCallTime = time; if (isInvoking) { if (timerId === undefined) { return leadingEdge(lastCallTime); } // Handle invocations in a tight loop. clearTimeout(timerId); console.log('setTimeout') timerId = setTimeout(timerExpired, wait); return invokeFunc(lastCallTime); } return result; } debounced.cancel = cancel; debounced.flush = flush; return debounced; }

ref

https://css-tricks.com/debouncing-throttling-explained-examples/

https://github.com/lodash/lodash/blob/4.7.0/lodash.js#L9840

https://jinlong.github.io/2016/04/24/Debouncing-and-Throttling-Explained-Through-Examples/

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