Question

I am trying to create a reusable function that checks if a variable is undefined or not. The strange thing is that it does not work when I pass the variable to the function to execute the code, but if I use the same logic outside of the function, it works. Is there any way to get this function isDefined to work?

//THIS WORKS AND RETURN FALSE
alert(typeof sdfsdfsdfsdf !== 'undefined');

//THIS GIVES AN ERROR, WHY?
//Uncaught ReferenceError: sdfsd is not defined 
function isDefined(value) {
        alert(typeof value !== 'undefined' && value !== null)
}

isDefined(sdfsd);

​Live example here (check the console for errors): http://jsfiddle.net/JzJHc/

Answer 1

You cannot use a variable that hasn’t been declared unless it’s in a typeof test

When you try to pass a variable that hasn’t been declared into a function, that is considered using that undeclared variable. You’ll notice that the error is in the caller, not inside isDefined

You need to run the check for

if (typeof sdsdsd !== 'undefined')

before you pass it into the function. Basically that means you can’t write a isDefined function that accepts undeclared variables. Your function can only work for undefined properties (which are OK to pass around)

However, I am curious, what is the real world case where you’re passing a variable that doesn’t exist? You should declare all your variables and they should exist already. Had you declared var sdsdsds it would exist with the value of undefined and your isDefined function would work just fine.