throttle without using setTimeout (only last run)

throttle.js

function throttle(fn, rate) {
    let oldTime;
    let timer;

    function helper(...args) {
        const now = Date.now();

        // first time: need to call fn once
        if(!oldTime || now - oldTime > rate) {
            const result = fn.apply(this, args);
            oldTime = now;
            clearTimeout(timer); // clear the last one if fn has already been called
            return result;
        }

        // last call if it hasn't reached to rate
        clearTimeout(timer);
        timer = setTimeout(() => {
            fn.apply(this, args);
            oldTime = Date.now(); // refresh oldTime
        }, rate - (now - oldTime));
    }

    return helper;
}

// Test 1:
// const doWithThrottle = throttle((i) => {
//     console.log(i)
// }, 5000);
// for(let i = 0; i< 100; i++) {
//     doWithThrottle(i);
// }

// output: 0, after 5 seconds, 99


// Test 2:
// let i = 0;
// const timer = setInterval(() => {
//     doWithThrottle(i++);
// }, 100);

// output: 0, 50, 100, ...