How to delete a variable?

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()!=''){
        userelm.ajaxloader(); //own function;
        delete code;
        delete userelm;

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

Delete a variable in JavaScript:


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:

  1. You are using a javascript interpreter or commandline.

  2. 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.

[email protected] ~ $ js

js> typeof boo

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

js> boo=5

js> typeof boo

js> delete(boo)

js> typeof boo

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:

    <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);

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!”.

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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

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

Delete does not delete a variable.

The delete operator removes a property from an object.

You can try:

code = null;

Try this in global scope:

x = 2;
delete window.x;

The following works in strict mode also:

"use strict";

var foo = 1;
// 1

foo = (function(){}());
// undefined


delete doesn’t affect variable names

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)

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

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

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

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

var obj = {
  'first' : 10;
//Following will print

delete obj.first;

//Following will print undefined/null/might give error

Bounus (for developers)

var item = 10;
delete item;
//Following will print 

//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

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

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?

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.

I was not successful with myvariable = null.

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

The answers/resolutions are collected from stackoverflow, are licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0 .

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