Javascript ||, aka Why Doesn't This Work?!

September 18, 2020

There is a piece of JavaScript syntax I, and others, seem to always want to work and it doesn’t.

const isThisOrThat = "that" === ("this" || "that")
// this is false

Why doesn’t this work? Let’s talk about it!

Inside the parens

If we go by order of operations, the inside expression evaluates first.

What does "this" || "that" mean in JavaScript?

I’ll be honest, this one breaks my math brain a bit. In math, a || b = b || a. The or operator is commutative, so order doesn’t matter. That’s not true in JavaScript, order matters a lot.

Let’s look at two examples.

const example = "" || "that"
// "that"

The first thing JavaScript does is check for the “existence” of the first value. JavaScript is checking for a falsey value, empty string is falsey. Since the first value doesn’t exist, it returns the second value.

This happens even if the second value is also falsey.

const example = "" || ""
// ""

So what happens if the first value is truthy, as in our original example?

const example = "this" || "that"
// "this"

example evaluates to "this".

Equality check

Now we start to realize why our expression up top doesn’t work. We can break it into two pieces.

const example = "this" || "that"
const isThisOrThat = "that" === example
// false

At the time that we’re checking equality, the string "that" is nowhere to be found.

What makes this strange is that order matters.

const example = "that" || "this"
const isThisOrThat = "that" === example
// true

Flipping the original or expression changes the resulting equality check.

Making sure this works

If we truly want to check both strings, we need to check equality twice.

const isThisOrThat = "that" === "this" || "that" === "that"
// true

Alternatively, we can use an array check. This one feels a bit more natural.

const isThisOrThat = ["that", "this"].includes("that")
// true

Or is weird

People use or a lot to flip values or set defaults, etc. It’s never been my favorite because of examples like the ones above. Once you start composing that expression it can confuse the issue.


A blog by Laurie Barth.
Teacher of all things tech.