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.