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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