why do we need "px"

Question

var pos = 0;
//our box element
var box = document.getElementById("box");
var t = setInterval(move, 10);

function move() {
if(pos >= 150) {
clearInterval(t);
}
else {
pos += 1;
box.style.left = pos+"px";
}
}
// why do we need to add "px" in the code

Answer 1

'px' is not mandatory as you could use other css units, but css unit for a css rule (and it is) is required (among wich 'px' is the most commonly used from Javascript dynamic style update, but not always the best suited -- ie. for responsive designs)

Answer 2

This code snippet changes the position defined in CSS. The position is given as an absolute position from left border with a value of previous position on page from left measured in pixels plus 1 per interval.