How to unset a JavaScript variable?

Question

I have a global variable in JavaScript (actually a window property, but I don\'t think it matters) which was already populated by a previous script but I don\'t

Answer 1

ECMAScript 2015 offers Reflect API. It is possible to delete object property with Reflect.deleteProperty():

Reflect.deleteProperty(myObject, 'myProp');
// it is equivalent to:
delete myObject.myProp;
delete myObject['myProp'];

To delete property of global window object:

Reflect.deleteProperty(window, 'some_var');

In some cases properties cannot be deleted (when the property is not configurable) and then this function returns false (as well as delete operator). In other cases returns true:

Object.defineProperty(window, 'some_var', {
    configurable: false,
    writable: true,
    enumerable: true,
    value: 'some_val'
});

var frozen = Object.freeze({ myProperty: 'myValue' });
var regular = { myProperty: 'myValue' };
var blank = {};

console.log(Reflect.deleteProperty(window, 'some_var')); // false
console.log(window.some_var); // some_var

console.log(Reflect.deleteProperty(frozen, 'myProperty')); // false
console.log(frozen.myProperty); // myValue

console.log(Reflect.deleteProperty(regular, 'myProperty')); // true
console.log(regular.myProperty); // undefined

console.log(Reflect.deleteProperty(blank, 'notExistingProperty')); // true
console.log(blank.notExistingProperty); // undefined

There is a difference between deleteProperty function and delete operator when run in strict mode:

'use strict'

var frozen = Object.freeze({ myProperty: 'myValue' });

Reflect.deleteProperty(frozen, 'myProperty'); // false
delete frozen.myProperty;
// TypeError: property "myProperty" is non-configurable and can't be deleted