[BUG]: TypeScript throws errors when adding [key: string]: string if other properties are not of type string

[RodrigoTomeES]

🔎 Search Terms

[key: string]: string override other types

🕗 Version & Regression Information

• This is an error with types

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.5#code/C4TwDgpgBAKhDOwoF4oG8BQUrAcAXOlAK4BOANoYqQJYB2A5lAL4DcWUA2gNYQhXBajALoChDdmyA

💻 Code

type Test = {
  test: { url: string };
  [key: string]: string;
};

🙁 Actual behavior

Throw error in test because is not a string

🙂 Expected behavior

Doesn't throw an error

Additional information about the issue

No response

[MartinJohns]

This is working as intended. The type of the index signature must be at least a union including all types of the known properties. This is documented: https://www.typescriptlang.org/docs/handbook/2/objects.html#index-signatures

If you really want this, you can use an intersection type instead: { test: { url: string } } & { [key: string]: string }
But be aware that this is not sound. It's easy to still assign a string to the property test.

[fatcerberus]

To represent this type properly you'd need #17867