通过 getElementById() 与 var speed 选择元素

Question

Could someone pls explain why selecting element through variable is faster than just using DOM query getElementById()? As I understand in the first case we ask interpreter to look through DOM tree for an element whose id is 'box' and once found interpreter returns its address. In the second case we store the address in variable first and then if we want to select the element we still need to lookup a var with the address that we need. So how looking up a var is faster than looking up the element itself?

//1
getElementById('box');

//2
var itemOne = getElementById('box');
itemOne;

Answer 1

When you run the getElementById it parses the DOM every time you run it. Once you allocate memory and store the result, the parsing is no longer needed, and the result is retrieved directly from the memory. I assume that's why it's faster.