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

@scunlife's answer will work, but technically it ought to be

delete window.some_var; 

delete is supposed to be a no-op when the target isn't an object property. e.g.,

(function() {
   var foo = 123;
   delete foo; // wont do anything, foo is still 123
   var bar = { foo: 123 };
   delete bar.foo; // foo is gone
}());

But since global variables are actually members of the window object, it works.

When prototype chains are involved, using delete gets more complex because it only removes the property from the target object, and not the prototype. e.g.,

function Foo() {}
Foo.prototype = { bar: 123 };
var foo = new Foo();
// foo.bar is 123
foo.bar = 456;
// foo.bar is now 456
delete foo.bar;
// foo.bar is 123 again.

So be careful.

EDIT: My answer is somewhat inaccurate (see "Misconceptions" at the end). The link explains all the gory details, but the summary is that there can be big differences between browsers and depending on the object you are deleting from. delete object.someProp should generally be safe as long as object !== window. I still wouldn't use it to delete variables declared with var although you can under the right circumstances.

Answer 2

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

Answer 3

some_var = null;

//or remove it..
delete some_var;

Answer 4

In addition to what everyone had written, also note that delete returns boolean. It can tell you if the delete was successful or not.

Testing on Chrome, everything except let was deletable. when delete returned true it actually removed them:

implicit_global = 1;
window.explicit_global = 1;
function_set = function() {};
function function_dec() { };
var declared_variable = 1;
let let_variable = 1;

delete implicit_global; // true, tested on Chrome 52
delete window.explicit_global; // true, tested on Chrome 52
delete function_set; // true, tested on Chrome 52
delete function_dec; // true, tested on Chrome 52
delete declared_variable; // true, tested on Chrome 52
delete let_variable; // false, tested on Chrome 78

Answer 5

You cannot delete a variable if you declared it (with var x;) at the time of first use. However, if your variable x first appeared in the script without a declaration, then you can use the delete operator (delete x;) and your variable will be deleted, very similar to deleting an element of an array or deleting a property of an object.

Answer 6

TLDR: simple defined variables (without var, let, const) could be deleted with delete. If you use var, let, const - they could not be deleted neither with delete nor with Reflect.deleteProperty.

Chrome 55:

simpleVar = "1";
"1"
delete simpleVar;
true
simpleVar;
VM439:1 Uncaught ReferenceError: simpleVar is not defined
    at <anonymous>:1:1
(anonymous) @ VM439:1
var varVar = "1";
undefined
delete varVar;
false
varVar;
"1"
let letVar = "1";
undefined
delete letVar;
true
letVar;
"1"
const constVar="1";
undefined
delete constVar;
true
constVar;
"1"
Reflect.deleteProperty (window, "constVar");
true
constVar;
"1"
Reflect.deleteProperty (window, "varVar");
false
varVar;
"1"
Reflect.deleteProperty (window, "letVar");
true
letVar;
"1"

FF Nightly 53.0a1 shows same behaviour.