Is the behavior of undefined different in Chrome?

Upon examining my Asp masterpage, I noticed the following code snippet:

<script>
    if (theForm !== undefined) { // <<== line 746: error
        theForm.onsubmit = ...bla bla... ;
    }
</script>

When checking the Chrome console, an error is displayed:

Uncaught ReferenceError: theForm is not defined   bla.aspx:746

I am now questioning whether this method of detecting an undefined name is incorrect, or if Chrome handles it differently?

(Please note that while Firefox console does not report an error, it still ceases to process subsequent JS code in this script block)

javascriptasp.netgoogle-chromeundefined

Answer №1

When you see the error message 

Uncaught ReferenceError: theForm is not defined

it means that

theForm is not declared

Why is this important to understand? Well, a variable can be undefined (but still declared) or it can simply not be declared at all.

For example:

  1. Declared but undefined

var foo;        // If a value was assigned (e.g.: var foo = 3), then it wouldn't be undefined
console.log(foo);

  1. Not declared (which will result in an error)

console.log(foo); // <-- foo was never declared

So how do we go about fixing this issue?

We can use typeof like so:

  console.log('Is undefined:', typeof foo === 'undefined');

Answer №2

It's important to understand the difference between a variable that has been declared but holds the value of undefined, and a variable that has never been declared at all. Based on your description, it sounds like the latter situation applies here. To determine if a variable has been declared, you can utilize the typeof operator.

if(typeof(someVariable) !== 'undefined'){ //typeof will return a string

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