How to delete a variable?

Question

Here is my javascript code, I want to delete the variable code so that it has value undefined.

var code = $(this).data('code');
var userelm = $(this);

Here I'm doing the check:

if($('.code-1').val()!='' && $('.code-2').val()!='' && $('.code-3').val()!=''){
    if(code==$('.code-1').val()+$('.code-2').val()+$('.code-3').val()){
        $('.overlay').remove();
        $('.code-box').remove();
        $('.close-lock').remove();
        userelm.ajaxloader(); //own function
        userelm.off();
        delete code;
        console.log(code);
        delete userelm;
    }
}

Why does this program not remove the code variable so it has the value undefined?

possible duplicate of [How to unset a Javascript variable?](http://stackoverflow.com/questions/1596782/how-to-unset-a-javascript-variable) — gpojd, Jun 06 '13 at 13:20

Eric Leschinski · Answer 1 · 2014-01-08T20:10:52.770

Delete a variable in JavaScript:

Summary:

The reason you are having trouble deleting your variable in JavaScript is because JavaScript won't let you. You can't delete anything created by the var command unless we pull a rabbit out our bag of tricks.

The delete command is only for object's properties which were not created with var.

JavaScript will let you delete a variable created with var under the following conditions:

You are using a javascript interpreter or commandline.
You are using eval and you create and delete your var inside there.

Demo on terminal, Use the delete boo or delete(boo) command. A demo with js command line terminal let you delete a variable.

el@defiant ~ $ js

js> typeof boo
"undefined"

js> boo
typein:2: ReferenceError: boo is not defined

js> boo=5
5

js> typeof boo
"number"

js> delete(boo)
true

js> typeof boo
"undefined"

js> boo
typein:7: ReferenceError: boo is not defined

If you MUST set your variable to undefined in JavaScript, you have one option:

Demo in javascript page: put this in myjs.html:

<html>
<body>
    <script type="text/JavaScript">
        document.write("aliens: " + aliens + "<br>");
        document.write("typeof aliens: " + (typeof aliens) + "<br>");
        var aliens = "scramble the nimitz";
        document.write("found some aliens: " + (typeof aliens) + "<br>");
        document.write("not sayings its aliens but... " + aliens + "<br>");
        aliens = undefined;
        document.write("aliens set to undefined<br>");
        document.write("typeof aliens: " + (typeof aliens) + "<br>");
        document.write("you sure they are gone? " + aliens);
    </script>
</body>
</html>

Open myjs.html with a browser, it prints this:

aliens: undefined
typeof aliens: undefined
found some aliens: string
not sayings its aliens but... scramble the nimitz
aliens set to undefined
typeof aliens: undefined
you sure they are gone? undefined

Warning When you set your variable to undefined you are assigning a variable to another variable. If someone poisons the well by running undefined = 'gotcha!', then whenever you set your variable to undefined, it becomes: "gotcha!".

How should we check if a variable has no value?

Use null instead of undefined like this:

document.write("skittles: " + skittles + "<br>");
document.write("typeof skittles: " + (typeof skittles) + "<br>");
var skittles = 5;
document.write("skittles: " + skittles + "<br>");
document.write("typeof skittles:" + typeof skittles + "<br>");
skittles = null;
document.write("skittles: " + skittles + "<br>");
document.write("typeof skittles: " + typeof skittles);

Which prints:

skittles: undefined
typeof skittles: undefined
skittles: 5
typeof skittles:number
skittles: null
typeof skittles: object

If you are not using strict, you can delete variables created like this:

<script type="text/JavaScript">
   //use strict
   a = 5;
   document.writeln(typeof a);        //prints number
   delete a;
   document.writeln(typeof a);        //prints undefined
</script>

but if you uncomment use strict, the javascript will not run.

Isn't null subject to the same issue as undefined? eg: null = 'gotcha'; — MikeSullivan, Aug 18 '15 at 19:35

score 25 · Answer 2 · answered Jun 06 '13 at 13:16

25

Delete does not delete a variable.

The delete operator removes a property from an object.

You can try:

code = null;

answered Jun 06 '13 at 13:16

gpojd

22,558
8
42
71

If you use this variable you get null. If you use a non-existing variable you get a ReferenceError. – Benur21 Nov 08 '22 at 15:46

score 8 · Answer 3 · edited May 05 '14 at 13:50

8

Try this in global scope:

x = 2;
delete window.x;

edited May 05 '14 at 13:50

answered Jan 23 '14 at 02:08

Tim W

81
1
1

score 8 · Answer 4 · answered Oct 10 '14 at 22:53

8

The following works in strict mode also:

"use strict";

var foo = 1;
console.log(foo);
// 1

foo = (function(){}());
console.log(foo);
// undefined

;)

answered Oct 10 '14 at 22:53

Argon

81
1
2

score 5 · Answer 5 · answered Jun 06 '13 at 13:18

delete doesn't affect variable names

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/delete

x = 42;         // creates the property x on the global object
var y = 43;     // declares a new variable, y
myobj = {
  h: 4,
  k: 5
};

delete x;       // returns true  (x is a property of the global object and can be deleted)
delete y;       // returns false (delete doesn't affect variable names)
delete Math.PI; // returns false (delete doesn't affect certain predefined properties)
delete myobj.h; // returns true  (user-defined properties can be deleted)

delete myobj;   // returns true  (myobj is a property of the global object, not a variable, so it can be deleted)

Even if you reset a variable that was created with var (by setting it to null), you still have the variable name in memory don't you? — Kokodoko, Feb 20 '14 at 14:21

score 2 · Answer 6 · answered Feb 08 '18 at 05:15

The others answer is quite simple and effective

code = null;
code = undefined;

But I want to share it's reason also, Why delete code; won't work

delete is meant to delete properties of objects only. It's never meant (developed) to delete variables.

var obj = {
  'first' : 10;
}
//Following will print
console.log(obj.first)

delete obj.first;

//Following will print undefined/null/might give error
console.log(obj.first)

Bounus (for developers)

var item = 10;
delete item;
//Following will print 
console.log(item);

//The reason for upper is when we declare variable with var it is of 
//primitive data type and delete won't work.


item = 10;
delete item;
//Following will give reference error
console.log(item);

//The reason for upper is when we declare variable without var it is a 
//property of global 'window' object so delete work.

score 1 · Answer 7 · answered May 05 '14 at 14:15

1

Variables created on the fly will be automatically deleted after there is no more reference to it.

(function(){
  var a = 1;
})(); //a available until here

answered May 05 '14 at 14:15

score 0 · Answer 8 · edited May 23 '17 at 12:02

0

because you write var for define the variable. if you use var can not delete it. you must be write this

userelm = null;

or don't write var. See this question: How to unset or remove a Javascript variable?

edited May 23 '17 at 12:02

Community

1
1

answered Mar 27 '16 at 15:07

peyman azizi

43
7

score 0 · Answer 9 · answered Apr 08 '16 at 14:16

0

you don't have to remove var , if you declared it in a function - when function ends, the inner var stops existing. You just don't have to declare variables globally.

answered Apr 08 '16 at 14:16

Vitalii

1

score 0 · Answer 10 · answered May 25 '16 at 18:44

0

I was not successful with myvariable = null.

But I was successful with myvariable = function(){};

answered May 25 '16 at 18:44

Michal Vlasák

247
2
14

How to delete a variable?

10 Answers10

Delete a variable in JavaScript:

How should we check if a variable has no value?

Linked