delete operator - JavaScript 编辑

The JavaScript delete operator removes a property from an object; if no more references to the same property are held, it is eventually released automatically.

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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

true for all cases except when the property is an own non-configurable property, in which case, false is returned in non-strict mode.

Exceptions

Throws TypeError in strict mode if the property is an own non-configurable property.

Description

Unlike what common belief suggests (perhaps due to other programming languages like delete in C++), the delete operator has nothing to do with directly freeing memory. Memory management is done indirectly via breaking references. See the memory management page for more details.

The delete operator removes a given property from an object. On successful deletion, it will return true, else false will be returned.

However, it is important to consider the following scenarios:

• If the property which you are trying to delete does not exist, delete will not have any effect and will return true.
• If a property with the same name exists on the object's prototype chain, then, after deletion, the object will use the property from the prototype chain (in other words, delete only has an effect on own properties).
• Any property declared with var cannot be deleted from the global scope or from a function's scope.
  • As such, delete cannot delete any functions in the global scope (whether this is part from a function definition or a function expression).
  • Functions which are part of an object (apart from the global scope) can be deleted with delete.
• Any property declared with let or const cannot be deleted from the scope within which they were defined.
• Non-configurable properties cannot be removed. This includes properties of built-in objects like Math, Array, Object and properties that are created as non-configurable with methods like Object.defineProperty().

The following snippet gives a simple example:

var Employee = {
  age: 28,
  name: 'abc',
  designation: 'developer'
}

console.log(delete Employee.name);   // returns true
console.log(delete Employee.age);    // returns true

// When trying to delete a property that does
// not exist, true is returned
console.log(delete Employee.salary); // returns true

Non-configurable properties

When a property is marked as non-configurable, delete won't have any effect, and will return false. In strict mode this will raise a TypeError.

var Employee = {};
Object.defineProperty(Employee, 'name', {configurable: false});

console.log(delete Employee.name);  // returns false

var, let, and const create non-configurable properties that cannot be deleted with the delete operator:

var nameOther = 'XYZ';

// We can access this global property using:
Object.getOwnPropertyDescriptor(window, 'nameOther');

// output: Object {value: "XYZ",
//                  writable: true,
//                  enumerable: true,
//                  configurable: false}

// Since "nameOther" is added using with the
// var keyword, it is marked as "non-configurable"

delete nameOther;   // return false

In strict mode, this would have raised an exception.

Strict vs. non-strict mode

When in strict mode, if delete is used on a direct reference to a variable, a function argument or a function name, it will throw a SyntaxError. Therefore, to avoid syntax errors in strict mode, you must use the delete operator in the form of delete object.property or delete object['property'].

Object.defineProperty(globalThis, 'variable1', { value: 10, configurable: true, });
Object.defineProperty(globalThis, 'variable2', { value: 10, configurable: false, });

// SyntaxError in strict mode.
console.log(delete variable1); // true

// SyntaxError in strict mode.
console.log(delete variable2); // false

function func(param) {
  // SyntaxError in strict mode.
  console.log(delete param); // false
}

// SyntaxError in strict mode.
console.log(delete func); // false

Cross-browser notes

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.

If you want to use an ordered associative array in a cross-browser environment, use a Map object if available, or simulate this structure with two separate arrays (one for the keys and the other for the values), or build an array of single-property objects, etc.

Examples

// Creates the property adminName on the global scope.
adminName = 'xyz';

// Creates the property empCount on the global scope.
// Since we are using var, this is marked as non-configurable. The same is true of let and const.
var empCount = 43;

EmployeeDetails = {
  name: 'xyz',
  age: 5,
  designation: 'Developer'
};

// adminName is a property of the global scope.
// It can be deleted since it is created without var,
// and is therefore configurable.
delete adminName;       // returns true

// On the contrary, empCount is not configurable
// since var was used.
delete empCount;       // returns false

// delete can be used to remove properties from objects.
delete EmployeeDetails.name; // returns true

// Even when the property does not exist, delete returns "true".
delete EmployeeDetails.salary; // returns true

// delete does not affect built-in static properties.
delete Math.PI; // returns false

// EmployeeDetails is a property of the global scope.
// Since it was defined without "var", it is marked configurable.
delete EmployeeDetails;   // returns true

function f() {
  var z = 44;

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

delete and the prototype chain

In the following example, we delete an own property of an object while a property with the same name is available on the prototype chain:

function Foo() {
  this.bar = 10;
}

Foo.prototype.bar = 42;

var foo = new Foo();

// foo.bar is associated with the
// own property.
console.log(foo.bar); // 10

// Delete the own property within the
// foo object.
delete foo.bar; // returns true

// foo.bar is still available in the
// prototype chain.
console.log(foo.bar); // 42

// Delete the property on the prototype.
delete Foo.prototype.bar; // returns true

// The "bar" property can no longer be
// inherited from Foo since it has been
// deleted.
console.log(foo.bar); // undefined

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 is not 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 is executed
}

If instead, you want to remove an array element by changing the contents of the array, use the splice() method. In the following example, trees[3] is removed from the array completely using splice():

var trees = ['redwood', 'bay', 'cedar', 'oak', 'maple'];
trees.splice(3,1);
console.log(trees); // ["redwood", "bay", "cedar", "maple"]

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• In depth analysis on delete
• Reflect.deleteProperty()
• Map.prototype.delete()