Why is null length returning 4?

General Discussions
, ,
1

Javascript check for not null Below is a code snippet, where we retrieve a form value. Before further processing, check if the value is not null.

var val = document.FileList.hiddenInfo.value; alert("val is " + val); // this prints null which is as expected if (val != null) { alert("value is "+val.length); // this returns 4 } else { alert(“value* is null”); } Any ideas why it happens so?

2

I’ve bumped into this a lot over my 6+ years working with JavaScript.

The thing is, when you see val.length returning a value—even when you’re expecting null—chances are, it’s actually an empty string ("") rather than null. This is a classic catch in JavaScript. Remember, empty strings aren’t the same as null, and val != null won’t catch them.

If you’re aiming for a cleaner check for javascript not null and not an empty string, try this:

if (val != null && val !== "") {
    alert("value is " + val.length);
} else {
    alert("value* is null or empty");
}

This way, you’re making sure the value is neither null nor an empty string before attempting to access its .length.

3

Yeah, totally agree with @kumari_babitaa —been debugging this kind of stuff for years.

Just to add a bit more context: in JavaScript, empty strings are considered falsy, but they still have a length of 0. So if you’re getting a length of 4, chances are your variable isn’t empty—it has 4 characters in it! This is why javascript not null checks need to go a step further and also account for non-empty strings.

Here’s a slightly more defensive approach:

if (val !== null && val !== "") {
    alert("value is " + val.length);
} else {
    alert("value* is null or empty");
}

This ensures that you don’t end up alerting a length from something unintended—like a leftover string.

4

I’ve run into this during testing edge cases—especially with form inputs.

Adding on to what @kumari_babitaa and @joe-elmoufak said, the key here is understanding that "" (empty string) is still a valid value and has a .length—even if it’s zero. And because JavaScript is loose with types, it’s easy to confuse that with null.

To make your condition truly robust when checking javascript not null and ensuring it’s not an empty value either, this snippet works well:

if (val != null && val !== "") {
    alert("value is " + val.length);
} else {
    alert("value* is null or empty");
}

One small tip: if this logic shows up often in your codebase, you might consider wrapping it in a utility function to keep things DRY.