delete operator

The delete operator removes a property from an object.

Syntax

delete expression 

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

delete object.property
delete object['property']

Parameters

object

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

property

The property to delete.

Return value

Throws in strict mode if the property is an own non-configurable property (returns false in non-strict). Returns true in all other cases.

Description

Unlike what common belief suggests, the delete operator has nothing to do with directly freeing memory (it only does indirectly via breaking references. See the memory management page for more details).

If the delete operator succeeds, it removes the property from the object entirely. However, if a property with the same name exists on the object's prototype chain, the object will inherit that property from the prototype.

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

delete can't remove certain properties of predefined objects (like Object, Array, Math etc). These are described in ECMAScript 5 and later as non-configurable.

Temporal dead zone

The "temporal dead zone" (TDZ), specified in ECMAScript 6 for const and let declarations, also applies to the delete operator. Thus, code like the following will throw a ReferenceError.

function foo() { 
  delete x;
  let x;
}

function bar() { 
  delete y; 
  const y; 
}

Examples

x = 42;         // creates the property x on the global object
var y = 43;     // creates the property y on the global object, and marks it as non-configurable
myobj = {
  h: 4,
  k: 5
};

// x is a property of the global object and can be deleted
delete x;       // returns true

// y is not configurable, so it cannot be deleted                
delete y;       // returns false 

// delete doesn't affect certain predefined properties
delete Math.PI; // returns false 

// user-defined properties can be deleted
delete myobj.h; // returns true 

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

function f() {
  var z = 44;

  // delete doesn't affect local variable names
  delete z;     // returns false
}

If the object inherits a property from a prototype, and doesn't have the property itself, the property can't be deleted by referencing the object. You can, however, delete it directly on the prototype.

function Foo(){}
Foo.prototype.bar = 42;
var foo = new Foo();

// returns true, but with no effect, 
// since bar is an inherited property
delete foo.bar;           

// logs 42, property still inherited
console.log(foo.bar);

// deletes property on prototype
delete Foo.prototype.bar; 

// logs "undefined", property no longer inherited
console.log(foo.bar);           

Deleting array elements

When you delete an array element, the array length is not affected. This holds even if you delete the last element of the array.

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
}

Specifications

Browser compatibility

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.

See also

• In depth analysis on delete