防抖函数

export function debounce(func, wait, immediate) {

let timeout, args, context, timestamp, result

const later = function() {

// 据上一次触发时间间隔

const last = +new Date() - timestamp

// 上次被包装函数被调用时间间隔 last 小于设定时间间隔 wait

if (last < wait && last > 0) {

timeout = setTimeout(later, wait - last)

} else {

timeout = null

// 如果设定为immediate===true，因为开始边界已经调用过了此处无需调用

if (!immediate) {

result = func.apply(context, args)

if (!timeout) context = args = null

}

}

}

return function(...args) {

context = this

timestamp = +new Date()

const callNow = immediate && !timeout

// 如果延时不存在，重新设定延时

if (!timeout) timeout = setTimeout(later, wait)

if (callNow) {

result = func.apply(context, args)

context = args = null

}

return result

}

}