Get all but last element in TypeScript

What if you want to get all elements but the last one inside an array in TypeScript ?

Well, it’s a JS feature, that should be relevant in any environment.

Naïve answer

The answer is

// remove only the last element of array a
let b = a.slice(0, -1);
// a is left as-is
// b is all but the last element

A more complete TS example would be

const a: number[] = [1, 2, 3, 4];
const result = a.slice(0, -1);
console.log(result); // Will output [1, 2, 3]
console.log(a); // Will output [1, 2, 3, 4]

However, I’m not 100% satisfied with that.

It works, and has no side-effect, which means you can confidently pipe the output.

But it’s not very readable.

Code for others

There is no a.getAllButLastElement, which would be a lot more convenient to read (and to remember!)

So if you want to avoid over-engineering, you have no other option than commenting directly the code :

const a: number[] = [1, 2, 3, 4];
const result = a.slice(0, -1); // remove last elt only
console.log(result);
console.log(a);

Mmh works, it is readable, but I’m still not 100% satisfied with that.

It’s not re-usable accross the project

Reusable all but last in TypeScript

So finally I would use something like this :

function allButLast<T>(arr: T[]): T[] {
  return arr.slice(0, -1);
}

const a: number[] = [1, 2, 3, 4];

const result = allButLast(a);

console.log(result);
console.log(a);