Behavior of delete operator in javascript

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

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

Tim Down · Accepted Answer · 2012-10-23 13:50:46Z

6

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.com/understanding-delete/

edited Oct 23, 2012 at 13:50

answered Aug 10, 2011 at 11:26

Tim Down

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Community · Accepted Answer · 2017-05-23 11:55:34Z

4

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).

edited May 23, 2017 at 11:55

CommunityBot

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answered Aug 10, 2011 at 10:13

Mathias Bynens

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1 Comment

Tim Down Over a year ago

Actually the first example doesn't work because y is declared using var and therefore has the [[DontDelete]] attribute (in ECMAScript 3: the equivalent attribute is [[Configurable]] is ECMAScript 5) set to true.

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