SOURCE

function throtle_1(fn, delay) {
    let timer;
    return function() {
        let context = this;
        let args = arguments;
        if(timer) {
            return
        }
        timer = setTimeout(function() {
            fn.call(context, args);
            timer = null;
        }, delay)
    }
}

function throtle_2(fn, delay) {
    let pre = 0;
    return function() {
        let context = this;
        let args = arguments;
        let now = new Date();
        if(now - pre > delay) {
            fn.call(context, args);
            pre = now;
        }
    }
}