Multiple index signatures allowing different index types

Suggestion

🔍 Search Terms

Multiple more than one index type property signature type specific member mapping TypeScript TS

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Put further restrictions on index signatures being mapped to member types, driven by the type of the index signature. Basically add multiple index signatures to an interface that map different type keys to different type members. Hugely increases the power of index signatures. Blocked by #26797.

📃 Motivating Example

This:

export type ColorIdentifier =
    `${string}.${string}`
    | 'background'
    | 'contrastActiveBorder' | 'contrastBorder' | 'focusBorder'
    | 'descriptionForeground' | 'errorForeground' | 'foreground';
export type ThemeScopeKey = `[${string}]`;
export interface IThemeScopedColorCustomizations {
    [colorId: ColorIdentifier]: string;
}
export interface IColorCustomizations {
    [colorIdOrThemeScope: ColorIdentifier | ThemeScopeKey]: IThemeScopedColorCustomizations | string;
}

Could be more accurately specified as:

export type ColorIdentifier =
    `${string}.${string}`
    | 'background'
    | 'contrastActiveBorder' | 'contrastBorder' | 'focusBorder'
    | 'descriptionForeground' | 'errorForeground' | 'foreground';
export type ThemeScopeKey = `[${string}]`;
export interface IThemeScopedColorCustomizations {
    [colorId: ColorIdentifier]: string;
}
export interface IColorCustomizations {
    [colorId: ColorIdentifier]: string;
    [themeScope: ThemeScopeKey]: IThemeScopedColorCustomizations;
}

💻 Use Cases

The vscode repo is full of interface collections that could make immediate use of this..

[MartinJohns]

Sounds like a duplicate of #26797.

#26797 is a blocking issue however I also don't want TypeScript to restrict object literal types (type = {} or interface {}) to one index signature ([key: <key type>]: <member type>).

[MartinJohns]

I'm not sure what you mean by "is a blocking issue".

I also don't want TypeScript to restrict object literal types (type = {} or interface {}) to one index signature ([key: ]: ).

The linked issue does not impose a restriction like that. It's even mentioned explicitly that multiple signatures should be possible:

since these are just index signatures, they can be added into any object type and in multiples.

Correct! I've got that PR version installed staged on my VM and multiple index signatures work.
🤦‍♂️ I think I was half-asleep when I read that PR lol