JavaScript in and delete operators

In this post I will describe in and delete JavaScript operators.

Operator in can be used to check if given object has given property:

var obj = {
	propA: 'xxx',
	propB: { someData: 101 }
console.log('propA' in obj); // true
console.log('notExisting' in obj); // false
obj.notExisting = 'xxx';
console.log('notExisting' in obj); // true

var propName = 'propA';
console.log(propName in obj); // true

While checking if object has given property in also checks prototype chain:

var proto = {
	propA: 'xxx'
var obj = Object.create(proto);
obj.propB = 'yyy';
console.log('toString' in obj); // true - from Object
console.log('propA' in obj); // true - from proto
console.log('propB' in obj); // true - own property

Adding properties to objects in JavaScript is easy, what’s about removing?

var obj = { };
obj.prop = 1;

// attempt to remove prop
obj.prop = undefined;

// assigning undefined to property doesn't remove it 
console.log('prop' in obj); // true
console.log(Object.keys(obj)); // ["prop"]

delete obj.prop;
// or delete obj['prop'];

// now property is gone
console.log('prop' in obj); // false 
console.log(Object.keys(obj)); // []

But we cannot delete anything from prototype chain:

var proto = { prop: 1 };
var obj = Object.create(proto);

console.log('prop' in obj); // true
console.log(Object.keys(obj)); // []

delete obj.prop;

// nothing changed
console.log('prop' in obj); // true
console.log(Object.keys(obj)); // []

Also in strict mode we cannot delete global functions and variables but we can delete properties of window object:

function globalFunc() { }
var globalVar = 1;
window.globalViaProp = 1;

(function () {
  'use strict';

  // "SyntaxError: Delete of an unqualified identifier in strict mode.
  // delete globalFunc;
  // delete globalVar;
  // delete globalViaProp;
  // "TypeError: Cannot delete property 'globalVar' of #<Window>
  // delete window.globalVar;
  // delete window.globalFunc;
  delete window.globalViaProp; // ok

As usual with JavaScript without strict mode you won’t get any of these errors.

And what if you want to make one of your own properties undeletable?
No problem just mark is as a non configurable prop:

(function () {
  'use strict';

  var obj = {};
  Object.defineProperty(obj, 'nonDeletable', {
    value: 'foo forever',
    writable: false,
    configurable: false, // cannot be deleted
    enumerable: true
  // strict mode again:
  // "TypeError: Cannot delete property 'nonDeletable' 
  // delete obj.nonDeletable;

The last thing about delete to remember is that it returns true when it manages to remove property and false otherwise. If you think that this may be useful, look at the example below:

var proto = { foo: 1};
var obj = Object.create(proto);
// property was not removed but we get true
console.log(delete; // true
console.log(; // 1
// property didn't exist but we get true again
console.log(delete; // true
console.log(; // undefined 

OK OK to be honest delete returns true when object doesn’t have own property after executing delete operation. In other words if delete returns true we may be certain that property name doesn’t appear in array returned by Object.keys().

The End


A Programmer, A Geek, A Human