Why Can’t You Delete var‑Declared Globals in JavaScript?

In JavaScript, there are two ways to declare global variables.

Explicit global variable declared with var

Implicit global variable declared without var

The difference lies in whether they can be removed with the delete operator.

Consider the following code:

var a = 'a'; // explicit global
b = 'b'; // implicit global
console.log(a); // a
console.log(b); // b
console.log(window.a); // a
console.log(window.b); // b

In JavaScript, global variables are properties of the global object ( window), so both can be accessed via window.

Attempt to delete them:

// Explicit global cannot be deleted
delete a; // returns false

// Implicit global can be deleted
delete b; // returns true

// Deletion results
console.log(typeof a); // string
console.log(typeof b); // undefined

The delete operator can remove an object’s property unless the property is non‑configurable; in strict mode it throws an exception.

This means variables declared with var are non‑configurable. You can verify this with Object.getOwnPropertyDescriptor:

Object.getOwnPropertyDescriptor(window, a);
// {value: "a", writable: true, enumerable: true, configurable: false}
Object.getOwnPropertyDescriptor(window, b);
// {value: "b", writable: true, enumerable: true, configurable: true}

The fundamental difference is that explicitly declared globals are non‑configurable and cannot be deleted.

Note that once a property’s configurable attribute is set to false, its descriptor cannot be changed, so you cannot make an explicit global deletable by altering its descriptor; conversely, you can make an implicit global non‑deletable by setting configurable to false:

b = 'b';
var descriptor = Object.getOwnPropertyDescriptor(window, b);
descriptor.configurable = false;
Object.defineProperty(window, b, descriptor);
delete b; // returns false