Can I set variables to undefined or pass undefined as an argument?

Question

I’m a bit confused about JavaScript’s undefined and null values.

What does if (!testvar) actually do? Does it test for u

Answer 1

JavaScript, how to set a variable to undefined on commandline:

Set a variable to undefined in the js javascript command line terminal that comes with Java on Ubuntu 12.10.

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 set a variable to undefined in a javascript:

Put this in myjs.html:

It prints this:

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

WARNING! When setting your variable to undefined you are setting your variable to another variable. If some sneaky person runs undefined = 'rm -rf /'; then whenever you set your variable to undefined, you will receive that value.

You may be wondering how I can output the undefined value aliens at the start and have it still run. It's because of javascript hoisting: http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html