New function isEmpty

[Alexnortung]

Let's say I want to get the first item in an array, but i want to make sure that the array is empty first. you can do it in these two ways

if (isNonEmpty(arr)) {
  const first = arr[0];
}

or

if (!isNonEmpty(arr)) {
  return;
}
const first = arr[0];

I would normally prefer the second option as it would allow for less nesting.

The problem is that !isNonEmpty(arr) doesn't really look good and is kinda weird to read (not is not empty). Here it would be much easier to just use isEmpty(arr). This would also read nicely if you want to use !isEmpty(arr) and still feels intuitive.

[uhyo]

You're right that isEmpty is more natural and erogonomic way of expressing this functionality. 😄

However, TypeScript doesn't allow us to do this. Fow now, this must be isNonEmpty in order to perform proper type narrowing.

Playground

function isEmpty<T>(array: readonly T[]): array is readonly [] {
  return array.length === 0;
}

function useArray(array: number[]) {
  if (isEmpty(array)) {
    return;
  }
  const first = array[0];
  //    ^?
  // oops, `first` is still `number | undefined` (with noUncheckedIndexedAccess)
}

function isNonEmpty<T>(array: readonly T[]): array is readonly [T, ...T[]] {
  return array.length !== 0;
}

function useArray2(array: number[]) {
  if (!isNonEmpty(array)) {
    return;
  }
  const first = array[0];
  //    ^?
  // wow, `first` is `number` here
}

[Alexnortung]

Ahh I see, that is quite annoying, but it makes sense :)