Spot the bug - #94: Moving Sprite Position

programming web dev
1

Find the bug in this animation loop.

let x = 0;
function tick() {
  x + 2;
  requestAnimationFrame(tick);
}
tick();

Reply with what is broken and how you would fix it.

2

x + 2; is just a math expression that gets thrown away. Nothing ever assigns the new value back into x, so it stays 0 forever.

Fix is to actually mutate x:

let x = 0;

function tick() {
  x += 2; // or: x = x + 2
  requestAnimationFrame(tick);
}

tick();
3

Spot the Bug answer: The statement x + 2; computes a value but never assigns it back to x, so x never changes.

The fix:
Change x + 2; to x += 2;

Why:
Expressions like x + 2 are evaluated and discarded unless assigned; without assignment x stays 0 forever, so the animation never actually moves. Using x += 2 updates x on each frame.