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
}
}