My answer to Facebook JavaScript interview question on dev.to.

animate.js

// https://dev.to/coderbyte/a-javascript-interview-question-asked-at-facebook-27po

var el = document.getElementById("myDiv");

function animate(el, totalTime, distance, startTime, position) {
    if (position >= distance) return;

    var expired = (new Date() - startTime) / 1000;
    position = expired * distance / totalTime * 1000; 

    el.style.left = position + 'px';

    requestAnimationFrame(function() {
        animate(el, totalTime, distance, startTime, position);
    });
}

animate(el, 2000, 300, new Date(), 50);