Microsoft/TypeScript

The second parameter type of `apply` should be `ArrayLike<T>` instead of `T[]`

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#61,835 opened on Jun 9, 2025

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 (1 comment) (1 reaction) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
Domain: lib.d.tsHelp WantedPossible Improvement

Description

🔎 Search Terms

String.prototype.apply

I found #33173, but the issue was closed without a reason. The problem still persists.

🕗 Version & Regression Information

At least since v3.3.3 according to the Playground.

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBArgDhgXhmApgdxgVQJZhQICCATqQIYCeAFANoAMAHgCwCMANMywExesBmALoBKANwAoAMpRSBAOYA6AGakQAWwDCACwqlNIACbpFFAA5mANrTBxLljvATigA

💻 Code

const u8 = new Uint8Array([0x41,0x42,0x43]);
String.fromCharCode.apply(null, u8);

🙁 Actual behavior

Argument of type 'Uint8Array<ArrayBuffer>' is not assignable to parameter of type 'number[]'.
  Type 'Uint8Array<ArrayBuffer>' is missing the following properties from type 'number[]': pop, push, concat, shift, and 3 more.(2345)

🙂 Expected behavior

no error

Additional information about the issue

This code works fine at runtime. ECMAScript spec does not require a genuine array at all. ArrayLike is sufficient.

Workarounds such as

String.fromCharCode.apply(null, Array.from(u8));

will impose runtime overheads.

Contributor guide

Tech stack
typescript
Domain
api
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript type systemArrayLike typelib/*.d.ts files
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75
Research direction
The contributor needs to find the type definition for `Function.prototype.apply` in the TypeScript standard library (likely in lib.es5.d.ts). The issue suggests changing the second parameter from `T[]` to `ArrayLike<T>`, referencing the ECMAScript spec. They should modify the relevant overload and verify with the provided playground example. Additionally, check for any other overloads that may need updates and run the existing test suite to ensure no regressions. The contributor can look at the closed issue #33173 for prior discussion.