节流函数

function throttle(fn, delay) {
    let lastTime;
    let timer;
    delay = delay || 300;
    return function() {
        let args = arguments;
        // 记录当前函数触发的时间
        let nowTime = Date.now();
        if (lastTime && nowTime - lastTime < delay) {
            clearTimeout(timer);
            timer = setTimeout(function() {
                // 记录上一次函数触发的时间
                lastTime = nowTime;
                // 修正this指向问题
                fn.apply(this, args);
            }, delay);
        } else {
            lastTime = nowTime;
            fn.apply(this, args);
        }
    };
}