Jeremy Iglehart Jeremy Iglehart - 6 months ago 16
Javascript Question

Is it better to return `undefined` or `null` from a javascript function?

I have a function which I have written which basically looks like this:

function getNextCard(searchTerms) {
// Setup Some Variables

// Do a bunch of logic to pick the next card based on termed passed through what I'll call here as 'searchTerms' all of this logic is omitted because it's not important for my question.
// ...

// If we find a next card to give, than give it
if (nextCardFound)
return nextCardFound;

// Otherwise - I'm returning undefined
return undefined;

Question: Would it be better to return "null" here?

I can pass whatever I want back - obviously... I just wasn't sure what is the best thing to use.

The code that calls this function knows how to deal with undefined (it actually won't ever really happen unless something goes horribly wrong)

The reason I'm asking this question is that I heard somewhere something that sounded like "Don't assign undefined to variables" or something - that it will make it harder to debug. So, the fact that I can see that
gets passed back tells me that the return is working - but basically function similar to


Mozilla Docs Didn't answer my question... google didn't either :\

This SO Question - was way too broad for what I'm trying to figure out here.


I will argue there is no best way, and even standard functions sometimes choose one or the other.

For example:

  • [[Prototype]]

    Ordinary objects have a [[Prototype]] internal slot, which determines from which other object they inherit from. Of course, there must be a way to say that an object does not inherit from any other one. In this case, "there is no such object" is represented using null.

  • Object.getOwnPropertyDescriptor

    It is expected to return a property descriptor, that is, an object which describes a property (e.g. value, writability, enumerability and configurability). However, the property may not exist. In this case, "there is no such property" is represented using undefined.

  • document.getElementById

    It is expected to return the element with the given ID. However, there might be no element with that ID. In this case, "there is no such element" is represented using null.

So just choose whatever you prefer or think makes more sense for your specific case.