How does this code work even the call to draw does not have arguments?

Question

Code from MDN
https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Asynchronous/Timeouts_and_intervals

let startTime = null;
function draw(timestamp) {
if(!startTime) {
startTime = timestamp;
}
currentTime = timestamp - startTime;
requestAnimationFrame(draw);
}
draw();

Answer 1

Because window.requestAnimationFrame(step) takes a callback step with timestamp set to the first augument.

Check out this simple code to see how window.requestAnimationFrame(step) get the timestamp when it run.

https://code.sololearn.com/WpQ8JrlfcLW7/?ref=app

Answer 2

Thanks Calviղ