函数的节流和函数的防抖

[JesseZhao1990]

函数的节流和函数的防抖都是优化高频率执行的函数的一种方法。我们知道浏览器中某些计算和处理要比其他昂贵很多。比如DOM的操作。当一个操作dom的函数高频率执行时，我们可以根据情况进行节流或防抖的处理。

节流函数

在每个时间段里，最多只允许运行一次。比如说resize调整窗口，在调整窗口的过程中，事件一直在高频率的触发，我们可以利用节流函数让其在一定的间隔时间段内最多触发一次。用一个形象的比喻吧。生产线上的啤酒瓶排成队往前跑，一个行动迟缓的树懒，每一段时间内只能拿到一个瓶子。

思路：第一次调用的时候记录一下时间戳，之后每次运行的时候都和上一次调用的时候的时间戳比较差值，当差值大于等于某个值的时候才执行。

function throttle (action, delay) {
  let timeout = null
  let lastRun = 0
  return function () {
    if (timeout) {
      return
    }
    let elapsed = Date.now() - lastRun
    let context = this
    let args = arguments
    let runCallback = function () {
      lastRun = Date.now()
      timeout = false
      action.apply(context, args)
    }
    if (elapsed >= delay) {
      runCallback()
    } else {
      timeout = setTimeout(runCallback, delay)
    }
  }
}

防抖函数

在高频调用中，只有足够的空闲时间，代码才会执行一次，常见的就是input的change事件，只有停顿输入的事件大于指定的时间，代码才会执行一次。用一个形象的比喻吧。生活中的声控灯，只要你不停的说话，灯就不会熄灭，只有你安静的时间大于一定的值时，灯才会熄灭。

思路：第一次调用的时候创建一个定时器，在指定的间隔时间之后运行代码，当第二次调用该函数时，清除前一次的定时器并另设置一个。如果前一个定时器已经执行了，这个操作就没任何意义，如果前一个定时器还没执行，其实就是将其替换为一个新的定时器。目的是只有在执行函数的请求停止了一段时间之后才执行。根据以上思路实现一个防抖函数。

function debounce(action, content, delay){
    let timeoutId = null;
    return function() {
        clearTimeout(timeoutId);
        timeoutId = setTimeout(function(){
            action.apply(content);
        }, delay);
    }
}

[JesseZhao1990]

lodash里的throttle 和 debounce 源码分析

debounce

import isObject from './isObject.js'
import root from './.internal/root.js'

/**
 * Creates a debounced function that delays invoking `func` until after `wait`
 * milliseconds have elapsed since the last time the debounced function was
 * invoked, or until the next browser frame is drawn. The debounced function
 * comes with a `cancel` method to cancel delayed `func` invocations and a
 * `flush` method to immediately invoke them. Provide `options` to indicate
 * whether `func` should be invoked on the leading and/or trailing edge of the
 * `wait` timeout. The `func` is invoked with the last arguments provided to the
 * debounced function. Subsequent calls to the debounced function return the
 * result of the last `func` invocation.
 *
 * **Note:** If `leading` and `trailing` options are `true`, `func` is
 * invoked on the trailing edge of the timeout only if the debounced function
 * is invoked more than once during the `wait` timeout.
 *
 * If `wait` is `0` and `leading` is `false`, `func` invocation is deferred
 * until the next tick, similar to `setTimeout` with a timeout of `0`.
 *
 * If `wait` is omitted in an environment with `requestAnimationFrame`, `func`
 * invocation will be deferred until the next frame is drawn (typically about
 * 16ms).
 *
 * See [David Corbacho's article](https://css-tricks.com/debouncing-throttling-explained-examples/)
 * for details over the differences between `debounce` and `throttle`.
 *
 * @since 0.1.0
 * @category Function
 * @param {Function} func The function to debounce.
 * @param {number} [wait=0]
 *  The number of milliseconds to delay; if omitted, `requestAnimationFrame` is
 *  used (if available).
 * @param {Object} [options={}] The options object.
 * @param {boolean} [options.leading=false]
 *  Specify invoking on the leading edge of the timeout.
 * @param {number} [options.maxWait]
 *  The maximum time `func` is allowed to be delayed before it's invoked.
 * @param {boolean} [options.trailing=true]
 *  Specify invoking on the trailing edge of the timeout.
 * @returns {Function} Returns the new debounced function.
 * @example
 *
 * // Avoid costly calculations while the window size is in flux.
 * jQuery(window).on('resize', debounce(calculateLayout, 150))
 *
 * // Invoke `sendMail` when clicked, debouncing subsequent calls.
 * jQuery(element).on('click', debounce(sendMail, 300, {
 *   'leading': true,
 *   'trailing': false
 * }))
 *
 * // Ensure `batchLog` is invoked once after 1 second of debounced calls.
 * const debounced = debounce(batchLog, 250, { 'maxWait': 1000 })
 * const source = new EventSource('/stream')
 * jQuery(source).on('message', debounced)
 *
 * // Cancel the trailing debounced invocation.
 * jQuery(window).on('popstate', debounced.cancel)
 *
 * // Check for pending invocations.
 * const status = debounced.pending() ? "Pending..." : "Ready"
 */
function debounce(func, wait, options) {
  let lastArgs,
    lastThis,
    maxWait,
    result,
    timerId,
    lastCallTime

  let lastInvokeTime = 0
  let leading = false
  let maxing = false
  let trailing = true

  // Bypass `requestAnimationFrame` by explicitly setting `wait=0`.
  const useRAF = (!wait && wait !== 0 && typeof root.requestAnimationFrame === 'function')

  if (typeof func != 'function') {
    throw new TypeError('Expected a function')
  }
  wait = +wait || 0
  if (isObject(options)) {
    leading = !!options.leading
    maxing = 'maxWait' in options
    maxWait = maxing ? Math.max(+options.maxWait || 0, wait) : maxWait
    trailing = 'trailing' in options ? !!options.trailing : trailing
  }

  function invokeFunc(time) {
    const args = lastArgs
    const thisArg = lastThis

    lastArgs = lastThis = undefined
    lastInvokeTime = time
    result = func.apply(thisArg, args)
    return result
  }

  function startTimer(pendingFunc, wait) {
    if (useRAF) {
      return root.requestAnimationFrame(pendingFunc)
    }
    return setTimeout(pendingFunc, wait)
  }

  function cancelTimer(id) {
    if (useRAF) {
      return root.cancelAnimationFrame(id)
    }
    clearTimeout(id)
  }

  function leadingEdge(time) {
    // Reset any `maxWait` timer.
    lastInvokeTime = time
    // Start the timer for the trailing edge.
    timerId = startTimer(timerExpired, wait)
    // Invoke the leading edge.
    return leading ? invokeFunc(time) : result
  }

  function remainingWait(time) {
    const timeSinceLastCall = time - lastCallTime
    const timeSinceLastInvoke = time - lastInvokeTime
    const timeWaiting = wait - timeSinceLastCall

    return maxing
      ? Math.min(timeWaiting, maxWait - timeSinceLastInvoke)
      : timeWaiting
  }

  function shouldInvoke(time) {
    const timeSinceLastCall = time - lastCallTime
    const timeSinceLastInvoke = time - lastInvokeTime

    // 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 === undefined || (timeSinceLastCall >= wait) ||
      (timeSinceLastCall < 0) || (maxing && timeSinceLastInvoke >= maxWait))
  }

  function timerExpired() {
    const time = Date.now()
    if (shouldInvoke(time)) {
      return trailingEdge(time)
    }
    // Restart the timer.
    timerId = startTimer(timerExpired, remainingWait(time))
  }

  function trailingEdge(time) {
    timerId = undefined

    // Only invoke if we have `lastArgs` which means `func` has been
    // debounced at least once.
    if (trailing && lastArgs) {
      return invokeFunc(time)
    }
    lastArgs = lastThis = undefined
    return result
  }

  function cancel() {
    if (timerId !== undefined) {
      cancelTimer(timerId)
    }
    lastInvokeTime = 0
    lastArgs = lastCallTime = lastThis = timerId = undefined
  }

  function flush() {
    return timerId === undefined ? result : trailingEdge(Date.now())
  }

  function pending() {
    return timerId !== undefined
  }

  function debounced(...args) {
    const time = Date.now()
    const isInvoking = shouldInvoke(time)

    lastArgs = args
    lastThis = this
    lastCallTime = time

    if (isInvoking) {
      if (timerId === undefined) {
        return leadingEdge(lastCallTime)
      }
      if (maxing) {
        // Handle invocations in a tight loop.
        timerId = startTimer(timerExpired, wait)
        return invokeFunc(lastCallTime)
      }
    }
    if (timerId === undefined) {
      timerId = startTimer(timerExpired, wait)
    }
    return result
  }
  debounced.cancel = cancel
  debounced.flush = flush
  debounced.pending = pending
  return debounced
}

export default debounce

throttle

import debounce from './debounce.js'
import isObject from './isObject.js'

/**
 * Creates a throttled function that only invokes `func` at most once per
 * every `wait` milliseconds (or once per browser frame). The throttled function
 * comes with a `cancel` method to cancel delayed `func` invocations and a
 * `flush` method to immediately invoke them. Provide `options` to indicate
 * whether `func` should be invoked on the leading and/or trailing edge of the
 * `wait` timeout. The `func` is invoked with the last arguments provided to the
 * throttled function. Subsequent calls to the throttled function return the
 * result of the last `func` invocation.
 *
 * **Note:** If `leading` and `trailing` options are `true`, `func` is
 * invoked on the trailing edge of the timeout only if the throttled function
 * is invoked more than once during the `wait` timeout.
 *
 * If `wait` is `0` and `leading` is `false`, `func` invocation is deferred
 * until the next tick, similar to `setTimeout` with a timeout of `0`.
 *
 * If `wait` is omitted in an environment with `requestAnimationFrame`, `func`
 * invocation will be deferred until the next frame is drawn (typically about
 * 16ms).
 *
 * See [David Corbacho's article](https://css-tricks.com/debouncing-throttling-explained-examples/)
 * for details over the differences between `throttle` and `debounce`.
 *
 * @since 0.1.0
 * @category Function
 * @param {Function} func The function to throttle.
 * @param {number} [wait=0]
 *  The number of milliseconds to throttle invocations to; if omitted,
 *  `requestAnimationFrame` is used (if available).
 * @param {Object} [options={}] The options object.
 * @param {boolean} [options.leading=true]
 *  Specify invoking on the leading edge of the timeout.
 * @param {boolean} [options.trailing=true]
 *  Specify invoking on the trailing edge of the timeout.
 * @returns {Function} Returns the new throttled function.
 * @example
 *
 * // Avoid excessively updating the position while scrolling.
 * jQuery(window).on('scroll', throttle(updatePosition, 100))
 *
 * // Invoke `renewToken` when the click event is fired, but not more than once every 5 minutes.
 * const throttled = throttle(renewToken, 300000, { 'trailing': false })
 * jQuery(element).on('click', throttled)
 *
 * // Cancel the trailing throttled invocation.
 * jQuery(window).on('popstate', throttled.cancel)
 */
function throttle(func, wait, options) {
  let leading = true
  let trailing = true

  if (typeof func != 'function') {
    throw new TypeError('Expected a function')
  }
  if (isObject(options)) {
    leading = 'leading' in options ? !!options.leading : leading
    trailing = 'trailing' in options ? !!options.trailing : trailing
  }
  return debounce(func, wait, {
    'leading': leading,
    'maxWait': wait,
    'trailing': trailing
  })
}

export default throttle