delete

Summary

The delete operator removes a property from an object.

Operator
Implemented in:	JavaScript 1.2
ECMAScript Edition:	ECMA-262 1st Edition

Syntax

delete expression

where expression should evaluate to a property reference, e.g.:

delete object.property
delete object['property']
delete object[index]
delete property // deletes properties of the global object, or, 
                // using the with statement, properties of the referenced object

If expression does not evaluate to a property, delete does nothing.

Parameters

object

The name of an object, or an expression evaluating to an object.

property

The property to delete.

index

An integer representing the array index to delete.

Returns

Returns false only if the property exists and cannot be deleted. It returns true in all other cases.

Description

delete is only effective on an object's properties. It has no effect on variable or function names.
While sometimes mischaracterized as global variables, assignments that don't specify an object (e.g. x = 5) are actually property assignments on the global object.

If the delete operator succeeds, it removes the property from the object entirely, although this might reveal a similarly named property on a prototype of the object.

delete can't remove certain properties of predefined objects (like Object, Array, Math etc). These are marked in the ECMA 262 specification as DontDelete.

x = 42;         // creates the property x on the global object
var y = 43;     // declares a new variable, y
myobj = {};
myobj.h = 4;    // creates property h on myobj
myobj.k = 5;    // creates property k on myobj

delete x;       // returns true  (x is a property of the global object and can be deleted)
delete y;       // returns false (delete doesn't affect variable names)
delete Math.PI; // returns false (delete doesn't affect certain predefined properties)
delete myobj.h; // returns true  (user-defined properties can be deleted)

with(myobj) { 
    delete k;   // returns true  (equivalent to delete myobj.k)
} 

delete myobj;   // returns true  (myobj is a property of the global object, not a variable, so it can be deleted)

You cannot delete a property on an object that it inherits from a prototype (although you can delete it directly on the prototype).

function Foo(){}
Foo.prototype.bar = 42;
var foo = new Foo();
delete foo.bar;           // but doesn't do anything
alert(foo.bar);           // alerts 42, property inherited
delete Foo.prototype.bar; // deletes property on prototype
alert(foo.bar);           // alerts "undefined", property no longer inherited

Deleting array elements

When you delete an array element, the array length is not affected. For example, if you delete a[3], a[4] is still a[4] and a[3] is undefined. This holds even if you delete the last element of the array (delete a[a.length-1]).

When the delete operator removes an array element, that element is no longer in the array. In the following example, trees[3] is removed with delete.

var trees = ["redwood","bay","cedar","oak","maple"];

delete trees[3];
if (3 in trees) {
    // this does not get executed
}

If you want an array element to exist but have an undefined value, use the undefined value instead of the delete operator. In the following example, trees[3] is assigned the value undefined, but the array element still exists:

var trees = ["redwood","bay","cedar","oak","maple"];
trees[3]=undefined;
if (3 in trees) {
    // this gets executed
}

Cross-browser issues

Although ECMAScript makes iteration order of objects implementation-dependent, it may appear that all major browsers support an iteration order based on the earliest added property coming first (at least for properties not on the prototype). However, in the case of Internet Explorer, when one uses delete on a property, some confusing behavior results, preventing other browsers from using simple objects like object literals as ordered associative arrays. In Explorer, while the property value is indeed set to undefined, if one later adds back a property with the same name, the property will be iterated in its old position--not at the end of the iteration sequence as one might expect after having deleted the property and then added it back.

So if you want to simulate an ordered associative array in a cross-browser environment, you are forced to either use two separate arrays (one for the keys and the other for the values), or build an array of single-property objects, etc.

External links

• In depth analysis on delete