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When strict equality goes wrong

  3 minute read

I recently saw this code in the wild. Names have been changed to protect those involved.

function makeFizzbobs (widget) {
    if (widget === null) {
        return {};
    } else {
        // Does something interesting with widget
        return 42;

I could imagine what the author of the code thought they were doing: they were guarding against an empty function parameter right? Cos null sounds like what you’d be left with if the user didn’t supply an argument like floop or 321. And all this linting software tells you to use triple equals instead of doubles so it must be right? User passes something in: carry on with the function, user doesn’t pass something to the function: we return an empty object. Job done.

🚨 No.

This is what actually happens:

makeFizzbobs(); // 42

Oh. So an empty argument isn’t the same thing as null? 🤔 No. No it’s not. Well what about an empty string, or maybe that undefined value, that sounds empty:

makeFizzbobs(''); // 42
makeFizzbobs(undefined); // 42
makeFizzbobs(false); // 42
makeFizzbobs(null); // {}

So it turns out that null only strictly equals itself, or null === null. Unsurprisingly, null !== undefined or any of the other values we tried above.

In fact, when you call the method with no arguments, widget gets the value undefined (add a console.log(widget) in the method to see for yourself). So how can we fix this function to guard against an empty argument? We could change the check in the if-statement from null to undefined, but then we’ll let nulls through instead.

Maybe you’d think that we could have a long if-clause, like:

if (widget === null || widget === undefined || widget === ... ) {

But there is a better way!

You can play along at home by whipping up some quick unit tests like I outlined in my other blog post. Define the makeFizzbobs function at the top of the test file and write some test cases.

Personally, I would short-circuit the function execution by checking for a falsey. There are 6 falsey values in JavaScript, and it’s likely that you aren’t interested in any of them:

  • false
  • NaN
  • 0
  • undefined
  • null

A falsey evaluates to false in an if-clause, which we can use like so:

function makeFizzbobs (widget) {
    if (widget) {
        // Do something exciting
        return 42;
    } else {
        return {};

You may notice that I’ve swapped the order around, so I’m dealing with desirable input first and then the falsey is dealt with in the else clause. This is because I think the if-clause reads a little clearer and is easier to comprehend this way, rather than having to reverse the logic in your head to work out what !widget means (I’m lazy, so what 😴).

So now the function works like:

makeFizzbobs(); // {}
makeFizzbobs(null); // {}
makeFizzbobs(''); // {}
makeFizzbobs(123); // 42
makeFizzbobs('abc'); // 42

Of course, this may not be suitable for your needs: perhaps you want to do something really interesting with an empty string '' or you just want to deal with 0 like any other number, or perhaps you really don’t want an empty array passed in, or Infinity, or the string "flibbertigibbet", but this is a good place to start in JavaScript’s loosely typed Wild West, and a good reminder that things are never what they seem with type checking in JavaScript.