Incorrectly allowing the use of methods that the return value of Object.groupBy does not have

[kurehajime]

🔎 Search Terms

Object.groupBy

🕗 Version & Regression Information

I tried with 5.2.2 and 5.6.0-dev.20240705
To see what is happening, you need to add "esnext" to lib.

⏯ Playground Link

https://www.typescriptlang.org/ja/play/?ts=5.6.0-dev.20240705#code/MYewdgzgLgBAhgJwTAvDA2gZgDQwIy4AsuBMArLgJy4BMAugFCiSwgBGAVqjAPKcCmwKADoA5ghABXAA4AhAJ4AKRAlyL+ASlQA+GPxgBSGDVQo0ABhgB+GACJ+AN35hbMAFx2QAEy+2NAbgZ2DmEoEABlKAQASzBRRS0AekSYABUAC31QAFtpaIAbfmQ4fPyQAHcIGCh06IhhGAAJCsci3Br9NglKour07qq4MD0kEARhIM4rUIio2PiklIy6mGgC-JhRaKcqjpguioheoZGJcaA

💻 Code

const arr = [3, 1, 4, 1, 5, 9, 2]
const obj = Object.groupBy(arr, (e) => e % 2 === 0 ? "even" : "odd");
obj.toString() // The compiler allows this. However, the browser throws an error.
obj?.toString() // This still gives the browser an error.

🙁 Actual behavior

The value returned by "Object.groupBy" is an object with null as prototype and does not have toString(). However, TypeScript compilers allow this.

🙂 Expected behavior

If a non-existent method such as toString is used in the value returned by "Object.groupBy", the compiler will detect it.

Additional information about the issue

No response

[kurehajime]

I noticed this after filing an Issue.
This code is more concise.

const obj = Object.create(null);
obj.toString()

[MartinJohns]

Essentially a duplicate of #1108.