Sean Massa's Labs

Should we prefer null or undefined in JavaScript?

2024-01-23 #software-engineering

JavaScript has two empty values where most languages have one (and some have zero!): null and undefined.

There certainly can be uses for more than one (like unknown empty vs known empty) and accommodations for this (like type coercion of falsey values). However, it adds complexity to using the base languages, runtimes, and libraries.

In an attempt to mitigate the complexity, I’ve long preferred to use null because it follows a “known empty” convention, which is more explicit. After digging into this in much more detail, my position has changed.


I read through many posts and historical references about why we have two values, the intent behind that, and the practicality of working with them today.

I found octogonz's reasoning to be the most concise and compelling:

Most programming languages have a "null" or "nil" value that functions...

  1. as the initial value for an uninitialized variable; and
  2. as the value of x.y or x["y"] when x has no such key; and
  3. as a special token that developers can assign to indicate an unknown or empty state.

In JavaScript, the undefined value fulfills roles (1), (2) and (3). JavaScript's null value is a redundant secondary token that can only be used for (3)

And sindresorhus's argument added that default parameters only work with undefined:

function multiply(a = 1, b = 2) {
  return a * b;

console.log(multiply(undefined, 3));
// 3

The use of null is largely a sentinel value to mean "known empty".


In the context of setting a linting requirement for a project, we have a few options:

  1. Do Nothing: anyone can use null or undefined as they see fit
  2. Prefer null: promote the use of null everywhere possible, but do not require it
  3. Prefer undefined: promote the use of undefined everywhere possible, but do not require it
  4. Require null: lint away all uses of undefined; work around any issues
  5. Require undefined: lint away all uses of null; work around any issues

In Theory

I think it's fair to say that the language didn't need these two similar concepts. We can certainly find specific cases where the distinction between them is meaningful, but in the end, I don't think it's often very useful.

Given how the language (and its programmers) treats these values, undefined is the true empty value of the language and null is the weird one out.

The theoretical ideal decision is therefore "(5) Require undefined".

In Practice

In practice, however, there are problems. Removing null means:

Developer education would also need to be produced for the team to explain the why, what, and how of it all.

My Preference

I’d like to choose "(5) Require undefined", but it's just not practical. That leaves "(3) Prefer undefined" as the best course of action.

Should we prefer <code>null</code> or <code>undefined</code> in JavaScript? | Sean Massa's Labs