Question

It seems in JavaScript you can’t delete function arguments but you
can delete global variables from a function.

Why this behavior?

var y = 1;
(function (x) { return delete y; })(1); // true

(function (x) { return delete x; })(1); // false

Answer 1

Edit: Both return false in normal use (i.e. not within the Firebug or browser console, which use eval()). See Tim Down’s answer (it should be the accepted one).