Add Array.prototype.findLast and Array.prototype.findLastIndex

[yume-chan]

lib Update Request

Configuration Check

My compilation target is ES5 and my lib is ESNext.

Missing / Incorrect Definition

Missing Array.prototype.findLast and Array.prototype.findLastIndex

Sample Code

[3, 2, 1].findLast(x => x > 2);

Documentation Link

Proposal: https://tc39.es/proposal-array-find-from-last/ (currently stage 3 4)
Can I Use: https://caniuse.com/mdn-javascript_builtins_array_findlast (Chrome and Safari already shipped it without flags)

[andrewbranch]

[Double checking with @DanielRosenwasser that stage 3 is the only criteria for adding to lib.esnext]

[DanielRosenwasser]

It is, but if I ask for stage 4 at an upcoming meeting we might as well add it to a stable version.

[fatcerberus]

Seems weird that Firefox hasn’t implemented it yet, that’s a bit unfortunate.

[Kingwl]

Seems weird that Firefox hasn’t implemented it yet, that’s a bit unfortunate.

Sorry....My fault.

[fatcerberus]

@Kingwl It’s your fault that Firefox doesn’t support this API? What?

[DanielRosenwasser]

@fatcerberus, the context is that @Kingwl is contributing the changeset to Firefox.

By the way, we reached Stage 4 today! 🎉🎉🎉

I believe that means we should add this to lib.es2023.array.d.ts.

[monikams]

With which version of typescript I could use the method findLast?

[juniorbotelho]

@monikams I found a temporally solution for this problem, just create a global.d.ts file in anywhere of your project and type this code:

export {}

declare global {
  interface Array<T> {
    findLastIndex(
      predicate: (value: T, index: number, obj: T[]) => unknown,
      thisArg?: any
    ): number
  }
}

Ready! You can use this approach without wrongs from your typescript linter.

[cobaltt7]

findLastIndex is in Node LTS v18.0.0

[RamonBalthazar]

Is adding these methods a breaking change? Why has this been moved to the 5.0 milestone? Thanks
@DanielRosenwasser

[andrewbranch]

5.0 is just the next release.

[jez9999]

Has this been added for 5.0? I'm really wanting to use findLastIndex and according to this, all modern browsers support it.

[DutchKevv]

Any updates?

Its a bit funky that we still have to include a custom typing for this...

[DanielRosenwasser]

It's in TypeScript 5.0 beta and the nightlies. The target release dates are available here.

[jez9999]

Well I upgraded my project to Typescript 5.0.2 today and hoped the errors would go away but nope, I still get:

Property 'findLastIndex' does not exist on type 'MoveDelta[]'. Do you need to change your target library? Try changing the 'lib' compiler option to 'es2023' or later.

If I try setting that in my tsconfig.json:

	"compilerOptions": {
		"lib":["es2023"],
	}

I get the build error:

Parsing error: Invalid value for lib provided: es2023

Can anyone help me out? What am I doing wrong?

[DanielRosenwasser]

Given the error message, it sounds like you need to upgrade your version of typescript-eslint.

[jez9999]

Given the error message, it sounds like you need to upgrade your version of typescript-eslint.

It seems to be TS itself giving the error. In VS code, the error is suffixed with "ts(2550)".

[DanielRosenwasser]

I'm not sure - if you can validate that your editor is using the latest version of TS (you can select the workspace version)

and that you see the same errors when all other extensions/plugins are disabled, then I would file a separate bug.

[jez9999]

Well it's not using that dev version but it is using the 5.0.2 workspace version. Should that have it?

[cristan]

It works on TypeScript 5 at least at the playground typescriptlang.org: example. It doesn't say much though: it also works just fine if you select Typescript 4.

Also note that you'll need at least Node.JS 18, as that's the lowest version that supports findLast (source).

[BR0kEN-]

It works only in TS 5 and only when the target is ESNext.

[juneate]

@cristan It works on TypeScript 5 at least at the playground typescriptlang.org

Incorrect, as you can see here:

As @BR0kEN- said, if you change the target in that environment, the issue disappears. But if you try to "Export" the example to CodeSandbox with the same settings, it triggers the issue again.

Tried also updating to the latest Typyscript 5.1 build, no change.

[cristan]

findLast was introduced in EcmaScript 2023. However, es2023 isn't supported in Typescript 5. Maybe that's it?

[shakedhagag]

Is there any progress with this issue?

[jacobbogers]

Is there any progress on this issue?

[RyanCavanaugh]

There's not progress because it was fixed in 2022. If you're not seeing findLast / etc, you have a configuration issue. Seek support at StackOverflow, Discord, etc.

[cristan]

Ah, it indeed does work, but the problem is indeed that findLast was introduced in EcmaScript 2023. You'll need Node.JS 18 or higher for that, but that that isn't much of a problem. TypeScript is slightly more problematic:

Setting the target to es2023 even in TypeScript 5.3.3 yields the same error I had before. However, findLast does work when setting the target to esnext ^_^

In the handbook, you'll see that es2023 is indeed not an option. It also states: "esnext is a moving target that may include support for Stage 3 proposals to ECMAScript modules". So I would much rather set the target explicitly to es2023 instead to this vague concept, but hey, it works.

[GoRRRRchik]

here is the solution

make the declaration under your code with findLastIndex

Array.prototype.findLastIndex = function(predicate) { for (let i = this.length - 1; i >= 0; i--) { if (predicate(this[i], i, this)) { return i; } } return -1; };

and it will fix your issue

[traez]

Compile Error Line 31: Char 32: error TS2550: Property 'findLastIndex' does not exist on type 'string[]'. Do you need to change your target library? Try changing the 'lib' compiler option to 'es2023' or later.

Got above on my LeetCode solution https://leetcode.com/problems/increasing-decreasing-string

MDN says

"Baseline 2022 Newly available Since August 2022, this feature works across the latest devices and browser versions. This feature might not work in older devices or browsers."

So I guess that's why. Leetcode yet to update their transpiler/compiler