• 防抖：事件持续触发，但只有当事件停止触发后n秒才执行函数。
• 节流：事件持续触发时，每n秒执行一次函数。

防抖

const debounce = function (func, delay, immediate = false) {
    let timer,
        result,
        callNow = true;
    const debounced = function () {
        const context = this;
        const args = arguments;
        if (timer) clearTimeout(timer);
        if (immediate) {
            if(callNow) result = func.apply(context, args);
            callNow = false;
            timer = setTimeout(() => {
                callNow = true; // 过n秒后才能再次触发函数执行。
            }, delay)
        } else {
            timer = setTimeout(() => {
                func.apply(context, args);
            }, delay)
        }
        return result;
    };
    debounced.cancel = function(){
        clearTimeout(timer);
        timer = null;
    }
}

节流

const throttle = function(func, delay) {
    let timer,
        prev = 0;
    return function(){
        const context = this;
        const args = arguments;
        const now = +new Date();
        const remaining = delay - (now - prev);
        if (remaining <= 0) {
            prev = now;
            func.apply(context, args);    
        } else if(!timer) {
            timer = setTimeout(() => {
                prev = +new Date();
                timer = null;
                func.apply(context, args);    
            }, remaining)    
        }
    }
}