Question

For example

var w = document.getElementById;
var b = w('header');
var c = w('footer');
var d = w('body');

Edit: Semicolons are another one of those big arguments. I thought I would edit the question for fun.

Edit: Responses to Andrey‘s comments found on his answer.

“How does copying reference make it more effective with JS compilers?”
Response: JS compilers are to shorten and/or obfuscate code. If there were 40 calls to document.getElementById(..), it would be much more compact if they called getById(..) which would be renamed to something like O(..).

“Also, when you handle html element events, you usually specify a js method, and inside the method you put the logic, not directly in the html event handlers – that is not required but a good practice”
Response: I know. But we have many many web systems and they rarely follow good practice completely.

“Also, using built in methods directly makes the code way more readable”
Response: Given these two examples, I think the latter is more readable

document.getElementById('total').value = document.getElementById('subtotal').value + document.getElementById('salestax').value - document.getElementById('discount').value
document.getElementById('yousaved').value = document.getElementById('discount').value / (document.getElementById('subtotal').value + document.getElementById('salestax').value)

or

var byId = document.getElementById
byId('total').value = byId('subtotal').value + byId('salestax').value - byId('discount').value
byId('yousaved').value = byId('discount').value / (byId('subtotal').value + byId('salestax').value)

Answer 1

Did you mean

var w = document.getElementById

?

Thsi link explains in detail why you shouldn’t do that: JavaScript function aliasing doesn't seem to work