节流：连续触发事件但在n秒中只执行一次函数。
function throttle(func, delay) {
let pre = 0;
return function() {
let context = this;
let args = arguments;
let now = new Date();
if (now - pre > delay) {
func(context, args);
pre = now;
}
}
}
function throttle(func, delay) {
let timer = null;
return function() {
let context = this;
let args = arguments;
if (timer) {
return;
}
timer = setTimeout(function() {
func(context, args);
timer = null;
}, delay)
}
}
防抖：触发事件后在n秒内函数只执行一次，如果n秒内又触发了事件，则会重新计算函数执行事件。
function debounce(func, delay) {
let timer = null;
return function() {
let context = this;
let args = arguments;
clearTimeout(timer);
timer = setTimeout(function() {
func(context, args);
}, delay)
}
}
