浅谈JS函数节流防抖

在前端开发中有一部分的用户行为会频繁的触发事件执行,而对于DOM操作、资源加载等耗费性能的处理,很可能导致界面卡顿,甚至浏览器的崩溃。函数节流(throttle)和函数防抖(debounce)就是为了解决类似需求应运而生的。

函数节流(throttle)

函数节流就是预定一个函数只有在大于等于执行周期时才执行,周期内调用不执行。好像水滴攒到一定重量才会落下一样。

场景:

  • 窗口调整(resize)
  • 页面滚动(scroll)
  • 抢购疯狂点击(mousedown)

实现:

function throttle(method, delay){
  var last = 0;
  return function (){
    var now = +new Date();
    if(now - last > delay){
      method.apply(this,arguments);
      last = now;
    }
  }
}

document.getElementById('throttle').onclick = throttle(function(){console.log('click')},2000);

underscore实现:

_.throttle = function(func, wait, options) {
  var context, args, result;
  var timeout = null;
  var previous = 0;
  if (!options) options = {};
  var later = function() {
    previous = options.leading === false ? 0 : _.now();
    timeout = null;
    result = func.apply(context, args);
    if (!timeout) context = args = null;
  };
  return function() {
    var now = _.now();
    if (!previous && options.leading === false) previous = now;
    //计算剩余时间
    var remaining = wait - (now - previous);
    context = this;
    args = arguments;
    //剩余时间小于等于0或者剩余时间大于等待时间(本地时间变动出现)
    if (remaining <= 0 || remaining > wait) {
      if (timeout) {
        clearTimeout(timeout);
        timeout = null;
      }
      previous = now;
      result = func.apply(context, args);
      if (!timeout) context = args = null;
    } else if (!timeout && options.trailing !== false) {
      timeout = setTimeout(later, remaining);
    }
    return result;
  };
};  

函数防抖(debounce)

函数防抖就是在函数需要频繁触发情况时,只有足够空闲的时间,才执行一次。好像公交司机会等人都上车后才出站一样。

场景:

  • 实时搜索(keyup)
  • 拖拽(mousemove)

实现:

function debounce(method, delay){
  var timer = null;
  return function(){
    var context = this,args = arguments;
    clearTimeout(timer);
    timer = setTimeout(function(){
      method.apply(context, args);
    },delay);
  }
}

document.getElementById('debounce').onclick = debounce(function(){console.log('click')},2000);

underscore实现:

_.debounce = function(func, wait, immediate) {
  var timeout, args, context, timestamp, result;
  var later = function() {
    var last = _.now() - timestamp;
    if (last < wait && last >= 0) {
      timeout = setTimeout(later, wait - last);
    } else {
      timeout = null;
      if (!immediate) {
        result = func.apply(context, args);
        if (!timeout) context = args = null;
      }
    }
  };
  return function() {
    context = this;
    args = arguments;
    timestamp = _.now();
    var callNow = immediate && !timeout;
    if (!timeout) timeout = setTimeout(later, wait);
    if (callNow) {
      result = func.apply(context, args);
      context = args = null;
    }
    return result;
  };
};  

      

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