The delete operator

JavaScriptJunior~1 min read

The delete operator in JavaScript is used to remove properties from objects (or elements from arrays). But it has several subtle nuances worth knowing. Let's go through it in detail.

Syntax

javascript

delete object.property

javascript

delete object['property']

Example with an object

javascript

const user = {
  name: 'Tim',
  age: 25
};

delete user.age;

console.log(user); // { name: 'Tim' }
console.log('age' in user); // false

The delete operator removes the property itself from the object, not just its value.

Return value

delete returns a logical value:

Case	Returns
Deletion succeeded	`true`
Deletion is impossible (e.g., the property is "non-configurable")	`false`

Example:

javascript

const obj = {};
Object.defineProperty(obj, 'x', { value: 10, configurable: false });

console.log(delete obj.x); // false
console.log(obj.x); // 10

Example with an array

javascript

const arr = [10, 20, 30];

delete arr[1];

console.log(arr); // [10, <1 empty item>, 30]
console.log(arr.length); // 3

Important: delete does not shift array elements, it simply removes the value, leaving a "hole" (empty).

If you need to remove an element while changing the array's length, use methods like:

javascript

arr.splice(1, 1); // removes the element and shifts the rest

Example with an object and `undefined`

The difference between delete and assigning undefined:

javascript

const user = { name: 'Tim', age: 25 };

user.age = undefined;
console.log(user);         // { name: 'Tim', age: undefined }
console.log('age' in user); // true (the key remains)

delete user.age;
console.log(user);         // { name: 'Tim' }
console.log('age' in user); // false (the key is removed)

delete removes the key entirely undefined - just clears the value

Example with a prototype

delete does not affect properties inherited through the prototype:

javascript

const person = { name: 'Alex' };
const user = Object.create(person);

console.log('name' in user); // true
delete user.name;            // won't delete from the prototype
console.log('name' in user); // true

It only removes the object's own properties.

Limitations

delete does not work on variables declared with var, let, const:

javascript

let x = 10;
console.log(delete x); // false
console.log(x); // 10

You can only delete a property of an object, not a variable.

In strict mode ("use strict"), an attempt to delete a non-deletable property will cause an error:

javascript

"use strict";
delete Object.prototype; // TypeError

Summary

What it does	Removes a property from an object
Works with	Objects, arrays
Returns	`true` or `false`
Does not work for	Variables (`var`, `let`, `const`)
Does not shift array elements	Leaves a "hole"
Safe for	An object's own properties

In short

javascript

const obj = { a: 1, b: 2 };
delete obj.a;

console.log(obj); // { b: 2 }
console.log('a' in obj); // false

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