SOURCE

function debounce(func, wait) {
	var lastCallTime // 最后点击时的时间
	var lastThis // 作用域
	var timerId // 定时器
	// 开启定时器
	function startTimer(wait) {
		return setTimeout(timerExpired, wait)
	}
	function remainingWait(time) {
		const timeSinceLastCall = time - lastCallTime
		const timeWaiting = wait - timeSinceLastCall
		return timeWaiting
	}

	// 判断是否可以调用函数
	function shouldInvoke(time) {
		return lastCallTime !== undefined && (time - lastCallTime >= wait)
	}
	// 判断是否执行func
	function timerExpired() {
		const time = Date.now()
		if (shouldInvoke(time)) {
			return invokeFun()
		}
		timerId = startTimer(remainingWait(time))
	}
	// 调用func
	function invokeFun() {
		// clearTimeout(timeId)
		timerId = undefined
		const thisArg = lastThis
		let result = func.call(thisArg)
		lastThis = undefined
		return result
	}
	
	

	function debounced() {
		lastCallTime = Date.now()
		lastThis = this
		if (timerId === undefined) {
			timerId = startTimer(wait)
		}
	}

	return debounced
}

window.addEventListener(
	'click',
	debounce(function(event) {
		var p = document.createElement('p')
		p.innerHTML = 'trigger'
		document.body.appendChild(p)
		return 'aaaa'
	}, 500)
)