throttle/debounce in JavaScript

debounce.js

const debounce = (func, wait, options = {
  leading: false,
  trailing: true
}) => {
  // same timer for leading and trailing
  let timer = null

  // special flag to skip the trailing call if leading is true
  let isCalledForLeading = false

  return function () {
    // when timer is up, trigger if trailing is true
    const delayed = () => {
      if (options.trailing && !isCalledForLeading) {
        func.apply(this, arguments)
      }
      timer = null
    }

    // if no timer , then a instant trigger
    if (options.leading && timer === null) {
      func.apply(this, arguments)
      isCalledForLeading = true
    } else {
      isCalledForLeading = false
    }

    // clear time and set a new one
    window.clearTimeout(timer)
    timer = setTimeout(delayed, wait)
  }
}

throttle.js

const throttle = (func, wait, options = {leading: true, trailing: true}) => {
  let timer = null
  let lastContext = null
  let lastArgs = null
  return function(...args) {

    // 1. if called within cool time, then store it for later call
    if (timer !== null) {
      lastContext = this
      lastArgs = args
      return
    }

    // 2. if other than cool time, execute it
    if (options.leading !== false) {
      func.call(this, ...args)
    } else {
      // save for trailing call if needed
      lastContext = this
      lastArgs = args
    }

    // 3. set a timeout to clear the cool, time
    // and run the stored context
    const timeup = () => {
      if (options.trailing !== false && lastArgs !== null) {
        func.call(lastContext, ...lastArgs)
        lastContext = null
        lastArgs = null
        timer = setTimeout(timeup, wait)
      } else {
        timer = null
      }
    }

    timer = setTimeout(timeup, wait)
  }
}