Suggestion

🔍 Search Terms

• keyof
• keyofStringsOnly

✅ 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

Since pattern literal types like `${number}` are valid as of #40598, I feel like there should be an option to make:

type ArrayLikeKeys = keyof ArrayLike<any>;

result in:

type ArrayLikeKeys = "length" | `${number}`;

The same should happen for any numeric property key:

interface Foo {
	1: "a";
	2: "b";
	3: "c";
}

// Currently is: : 1 | 2 | 3
// Should be: "1" | "2" | "3"
type KeyOfFoo = keyof Foo;

// Currently is: never
type StrictKeyOfFoo = (keyof Foo) & (string | symbol);

like with:

interface Foo {
	"1": "a";
	"2": "b";
	"3": "c";
}

// Is: "1" | "2" | "3"
type KeyOfFoo = keyof Foo;

📃 Motivating Example

This makes keyof and numeric index signatures match runtime behaviour.

💻 Use Cases

Currently, it’s necessary to use the strictKeyof and StrictPropertyKey helpers:

type strictKeyof<T> = keyof T extends infer K
	? (K extends number ? `${K}` : K)
	: never;

type StrictPropertyKey = string | symbol;

Relevant issues:

• Mapped tuples types iterates over all properties #27995
• fix(lib/es2015): Fix definition of ProxyHandler #35594 (comment), fix(lib/es2015): Fix definition of ProxyHandler #35594 (comment), fix(lib/es2015): Fix definition of ProxyHandler #35594 (comment)
• keyof object is number instead of string  #41669
• fix(lib/es2015): Fix types of Reflect methods #41987
• Improve support for numeric string types #48837
• Design Meeting Notes, 4/27/2022 #48957

This will most likely depend on #26797, so that treating numeric index signatures as numeric pattern literal index signatures is valid:

interface ArrayLike<T> {
	readonly [index: `${number}`]: T;
	readonly length: number;
}