microsoft / microsoft/TypeScript
The second parameter type of `apply` should be `ArrayLike<T>` instead of `T[]`
- Dominant language
- Go
- Stars
- 111k
- Forks
- 14.3k
- PR merge metrics
- PR metrics pending
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
```ts
const u8 = new Uint8Array([0x41,0x42,0x43]);
String.fromCharCode.apply(null, u8);
```
### 🙁 Actual behavior
```
Argument of type 'Uint8Array' is not assignable to parameter of type 'number[]'.
Type 'Uint8Array' 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](https://262.ecma-international.org/#sec-function.prototype.apply) does not require a genuine array at all. ArrayLike is sufficient.
Workarounds such as
```ts
String.fromCharCode.apply(null, Array.from(u8));
```
will impose runtime overheads.
Contributor guide
Research direction
Start with the linked TypeScript Playground reproduction and inspect the library declaration for Function.apply, focusing on the second parameter type. Confirm that the declaration accepts the Uint8Array example without error while preserving the existing apply behavior.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 2/5
- Estimated time
- 1-3 hours
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 48/100