js函数防抖、节流实现

React中使用防抖函数和节流函数

在React事件调用时，React传递给事件处理程序是一个合成事件对象的实例。SyntheticEvent对象是通过合并得到的。 这意味着在事件回调被调用后，SyntheticEvent 对象将被重用并且所有属性都将被取消。 这是出于性能原因。 因此，您无法以异步方式访问该事件。React合成事件官方文档

所以在用防抖或节流函数封装时，异步方式访问事件对象出现问题。解决的方法如下：

方法一：调用合成事件对象的persist()方法 event.persist && event.persist() //保留对事件的引用
方法二：深拷贝事件对象 const event = e && {...e} //深拷贝事件对象

function debounce(func, wait=500) {
    let timeout;  // 定时器变量
    return function(event){
        clearTimeout(timeout);  // 每次触发时先清除上一次的定时器,然后重新计时
        event.persist && event.persist()   //保留对事件的引用
        //const event = e && {...e}   //深拷贝事件对象
        timeout = setTimeout(()=>{
            func(event)
        }, wait);  // 指定 xx ms 后触发真正想进行的操作 handler
    };
}

防抖debounce

防抖 Debounce 多次触发，只在最后一次触发时，执行目标函数。

函数防抖就是，延迟一段时间再执行函数，如果这段时间内又触发了该函数，则延迟重新计算。

应用场景

（1）通过监听某些事件完成对应的需求，比如：

通过监听 scroll 事件，检测滚动位置，根据滚动位置显示返回顶部按钮
通过监听 resize 事件，对某些自适应页面调整DOM的渲染（通过CSS实现的自适应不再此范围内）
通过监听 keyup 事件，监听文字输入并调用接口进行模糊匹配

（2）其他场景

表单组件输入内容验证
防止多次点击导致表单多次提交

...

简单实现

function debounce(fn, wait) {
  let t
  return () => {
    let context = this
    let args = arguments
    if (t) clearTimeout(t)
    t= setTimeout(() => {
        fn.apply(context, args)
    }, wait)
  }
}

完整实现

function debounce(func, wait, immediate) {
    let time;
    let debounced = function() {
        let context = this;
        if(time) clearTimeout(time);

        if(immediate) {
            let callNow = !time;
            if(callNow) func.apply(context, arguments);
            time = setTimeout(
                ()=>{time = null} //见注解
            , wait)
        } else {
            time = setTimeout(
                ()=>{func.apply(context, arguments)}
            , wait) 
        }
    };

    debounced.cancel = function() {
        clearTimeout(time);
        time = null
    };
    return debounced
}

// underscore.js debounce

//
// Returns a function, that, as long as it continues to be invoked, will not
// be triggered. The function will be called after it stops being called for
// N milliseconds. If `immediate` is passed, trigger the function on the
// leading edge, instead of the trailing.

_.debounce = function(func, wait, immediate) {
  var timeout, args, context, timestamp, result;

  // 处理时间
  var later = function() {
    var last = _.now() - timestamp;

    if (last < wait && last >= 0) {
      timeout = setTimeout(later, wait - last); // 10ms 6ms 4ms
    } else {
      timeout = null;
      if (!immediate) {
        result = func.apply(context, args);
        if (!timeout) context = args = null;
      }
    }
  };

react中调用方法

this.handleGetCustomerNameList = debounce(this.handleGetCustomerNameList.bind(this), 500);

节流 throttle

节流：函数间隔一段时间后才能再触发，避免某些函数触发频率过高，比如滚动条滚动事件触发的函数。

### 简单实现
function throttle (fn, wait, mustRun) {
  let start = new Date()
  let timeout
  return () => {
    // 在返回的函数内部保留上下文和参数
    let context = this
    let args = arguments
    let current = new Date()

    clearTimeout(timeout)

    let remaining = current - start
    // 达到了指定触发时间，触发该函数
    if (remaining > mustRun) {
      fn.apply(context, args)
      start = current
    } else {
      // 否则wait时间后触发，闭包保留一个timeout实例
      timeout = setTimeout(fn, wait);
    }
  }
}

完整实现

function throttle(func, wait, options) {
    let time, context, args, result;
    let previous = 0;
    if (!options) options = {};

    let later = function () {
        previous = options.leading === false ? 0 : new Date().getTime();
        time = null;
        func.apply(context, args);
        if (!time) context = args = null;
    };

    let throttled = function () {
        let now = new Date().getTime();
        if (!previous && options.leading === false) previous = now;
        let remaining = wait - (now - previous);
        context = this;
        args = arguments;
        if (remaining <= 0 || remaining > wait) {
            if (time) {
                clearTimeout(time);
                time = null;
            }
            previous = now;
            func.apply(context, args);
            if (!time) context = args = null;
        } else if (!time && options.trailing !== false) {
            time = setTimeout(later, remaining);
        }
    };
    return throttled;
}

// underscore.js throttle

// Returns a function, that, when invoked, will only be triggered at most once
// during a given window of time. Normally, the throttled function will run
// as much as it can, without ever going more than once per `wait` duration;
// but if you'd like to disable the execution on the leading edge, pass
// `{leading: false}`. To disable execution on the trailing edge, ditto.

_.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;
    var remaining = wait - (now - previous);
    context = this;
    args = arguments;
    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;
  };
};

react中调用方法

this.handleGetCustomerNameList = throttle (this.handleGetCustomerNameList.bind(this), 500);

参考文章
JavaScript 函数防抖（debounce）的实现
React中事件节流防抖
在React、Vue和小程序中使用函数节流和函数防抖

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