Javascript - succinct way to assign another value to variable if first value is not truthy

Question

I have a function that may return a value or may return null and I want to assign it to a variable. So at the moment I've got

someVar = (someFun())
  ? someFun()
  : "foo";

Is there a shorterer way of doing it, where I don't have to call the function twice, or add an extra variable like:

funResult = someFun();
someVar = (funResult)
  ? funResult
  : "foo";

Basically, something like:

someVar = someFun() | "foo"

TIL what "null coalescing" means. – stib Mar 21 '18 at 05:15 — stib, Mar 21 '18 at 05:15

Pointy · Accepted Answer · 2018-03-21T03:05:03.677

1

The idiomatic way is

someVar = someFun() || "foo";

The || operator will yield the value of the left-hand expression if it's truthy. If not, it will move on to evaluate the right-hand side, and return its value.

For those unfamiliar with the term "truthy", in JavaScript it means values that are not considered by the language to be implicitly false. The implicitly false values are 0, NaN, false (of course), "", null, and undefined. Any reference to an object, even if it's completely empty, is considered to be "truthy".

In your specific example, the || approach is to be preferred because it means you only call someFun() once.

edited Mar 21 '18 at 03:05

answered Mar 21 '18 at 01:31

Pointy

405,095
59
585
614

You might want to emphasise that "not truthy" is more than just `null`. – Bergi Mar 21 '18 at 01:46
@Bergi I thought about that but the OP seemed to "get it"; however for posterity that's definitely a good idea. – Pointy Mar 21 '18 at 03:02
I knew that syntax was familiar. I was just missing a | – stib Mar 21 '18 at 05:18

Javascript - succinct way to assign another value to variable if first value is not truthy

1 Answers1