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

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

• javascript

• 1 Both return false in normal use (i.e. not within the Firebug or browser console, which use eval()). See my answer. – Tim Down Commented Aug 10, 2011 at 13:14

2 Answers 2

Actually, neither should return true, and indeed they don't in Firefox or Chrome (untested in other browsers). I imagine you tested this with Firebug or another browser console, which changes things due to the console using eval(). delete only deletes properties of an object and cannot normally delete a variable declared using var, whatever the scope.

Here's an excellent article by Kangax on the subject: http://perfectionkills./understanding-delete/

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).