Java Script: Array.length expression evaluate to boolean

Question

How to check array length, if I need the expression to evaluate to boolean value as a result? For example:

var myArray = [];
if(!myArray.length){
  return;
}

Or:

vr myArray = [];
if(myArray.length == 0){
  return;
}

Both of the examples work, however I’d like to understand what is the difference?

0 is a falsy value, and as such, if a condition evaluates to 0, it is considered false. The preceding ! operator (semantically "not") inverts the boolean to true. — Tyler Roper
– Tyler Roper, Commented Jun 30, 2017 at 15:59
array length is 0. but since 0 is false, condition is true in 1st. also null,"",undefined etc are false — Slim Shady
– Slim Shady, Commented Jun 30, 2017 at 16:00
It's important to understand the concept of truthy and falsy in JavaScript. I think the linked resources will help... — War10ck
– War10ck, Commented Jun 30, 2017 at 16:01
There is no best solution - but I'll suggest you better. Don't use == operator, in this case - length can be just number and you are comparing with a number so use === If you ask me, I would go with !array.length. And always use === instead of == (search for the book: JavaScript: The Good Parts) — Darkves
– Darkves, Commented Jun 30, 2017 at 16:07

cнŝdk · Accepted Answer · 2017-06-30 16:03:03Z

3

Here !myArray.length will return true in two cases:

If myArray.length===0 because 0 is a falsy value, so !0 will return true.
If myArray.length is undefined, in other words myArray is not an array so it doesn't have the length property, where undefined is falsy value too.

And the first one explains why both !myArray.length and myArray.length==0 are equivalent in your case.

answered Jun 30, 2017 at 16:03

cнŝdk

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Comments

gobaldia · Accepted Answer · 2017-06-30 16:05:21Z

0

This is because length function returns a Number and when you use Number as condition, 0 and NaN are evaluated as false (in other case is evaluated as true).

answered Jun 30, 2017 at 16:05

gobaldia

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Comments

Ben Racicot · Accepted Answer · 2023-04-22 14:58:22Z

0

If you have an empty array, that is an array with 0 length,

single negation with ! returns a misleading boolean.

arr = []

!arr.length // true - because `0` is `false`, `!` negates `false` to `true`

// double negate it for clear boolean
console.log(!!arr?.length, arr?.length); // false 0

Truthy example

arr = ['','','']

// array with values 
console.log(!!arr?.length, arr?.length); // true 3

Make sure to check if .length exists with optional chain ?

if (!!arr?.length) console.log(!!arr?.length);

Now you can reliably use .length as a boolean to check for empty arrays.

Note: Double negation has caveats like ESLint's no-extra-boolean-cast

answered Apr 22, 2023 at 14:58

Ben Racicot

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