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Javascript null check - or is it?

I'm back to the blog! After so many years it seems that the fire is still there and I felt the need to share little something. Without further rumble, let's jump into one of the situations that I find a lot on the job: checking for null on Javascript.
Here is the kind of code that has caused me much pain:

if (shipType)
do something
view raw not-operator.js hosted with ❤ by GitHub
 The intent of this is to only do stuff if shipType has a value. I always try to avoid this piece of code and use something more explicit, even when I know there is no problem with the code now.

But why so picky? Checking for null this way has a few problems. Here is a snippet demonstrating:


var shipType = undefined;
if (shipType) console.log("this is never shown. CORRECT");
shipType = null;
if (shipType) console.log("this is never shown. CORRECT");
shipType = "abc";
if (shipType) console.log("This is shown! CORRECT");
shipType = 0;
if (shipType) console.log("this is never shown, BUT IT SHOULD");
shipType = "";
if (shipType) console.log("this is never shown, BUT IT SHOULD");
view raw the-mighty-javascript-null-check.js hosted with ❤ by GitHub
Notice how, if the value of shipType is a number and is 0, it is never printed. This kind of bug becomes very common because C# uses integers by default as an underlying type for enums, and 0 is the default value for it.

The reason is that null is a falsy value - if you force it to be evaluated as a boolean it will return false. Falsy values are what allows you to write that syntax in the first place. Well, guess what? 0 is also a falsy value! So is the empty string value "". 

So, next time you want to check for null, do so explicitly to avoid these kinds of bugs.

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