Tuples: maintaining type-param in the type of it's items

🔍 Search Terms

for-of, T[number], tuple (I couldn't find more specific search terms).

✅ Viability Checklist

• 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

When a function takes an array/tuple as a type-parameter, e.g.

function foo<T extends readonly unknown[]>(items: T): void;

When accessing the items of this array (items[0]), they are "resolved" to the type defined for the generic, e.g., unknown in this example, but should be kept unresolved so that they show up as T[number] | undefined.

This becomes an issue once a function takes more than one array like this, for example, if it wants to use the tuple/array shape as a param in the output. This can lead to real runtime errors where TypeScript isn't providing enough safety.

This is also relevant for for-of loops iterating over items

for (const item of items) {
  console.log(item);
  //          ^? unknown, can be T[number]
}

📃 Motivating Example

The following example passes typescript checking but fails in runtime and throws exceptions when trying to access methods of strings on a number, and vice-versa.

type DoubleMap<
  T extends readonly unknown[],
  S extends readonly unknown[],
  RT,
  RS,
> = [...{ [I in keyof T]: RT }, ...{ [I in keyof S]: RS }];

function doubleMap<
  T extends readonly unknown[],
  S extends readonly unknown[],
  RT,
  RS,
>(
  t: T,
  s: S,
  mapperT: (item: T[number]) => RT,
  mapperS: (item: S[number]) => RS,
): DoubleMap<T, S, RT, RS> {
  const output: (T[number] | S[number])[] = [];

  for (const itemT of t) {
    // Oops, we got the mappers wrong, we are calling mapperS with an item from T
    output.push(mapperS(itemT));
  }

  for (const itemS of s) {
    // ...And here we are calling mapperT with an item from S
    output.push(mapperT(itemS));
  }

  // @ts-expect-error [ts2322] -- TypeScript can't tell we finished building the output, this is fine...
  return output;
}

const result = doubleMap(
  //  ^? [boolean, boolean, boolean, number, number, number]
  ["a", "b", "c"] as const,
  [1, 2, 3] as const,
  (item) => item.startsWith("a"),
  (item) => item.toPrecision(1),
);

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&noUnusedLocals=true&noUnusedParameters=true&noPropertyAccessFromIndexSignature=true&noImplicitOverride=true&noFallthroughCasesInSwitch=true&exactOptionalPropertyTypes=true#code/C4TwDgpgBAIg9gVwEYBsIFkCGYA8AoKKAFSggA9gIA7AEwGcoAnCTGuKlEKBKgayrgB3KgG0AugBoCUAMqkK1ekxZsOXHvyGjJ0gEpEphXTKkA+KAF4oIgHR2A3tYCSUAJZUovCCDgAzYmIAXFD6UAC+ElB2No4iLu6e3n6yQSFyYWIA3Hh4vjwAxsCu7FBsyGhYuNIk5JS0DMys7JzcfALC4oay8nVKjaotGu3aXfqjJnimABTSwMEG0nTBE4QAttiQjETBU66Uq-MiVAirSBCMYgCUluZj0utgmzI7exAHskcnZxfXFrcTl2C8HKGGwOAMskiYzS5ns0ny7DowCgiGAYAQcygUyIn1O5zEUAAPh9jnifuJLNYsjlCL44IwsQiqEi3PsSMlgNc4YRCAB6XlQADycDAdEigmgAHM4MjgAALaAPTYMQSMdiS8XQTDMKD5TAoFDuSVQJXnOSCPZyqCYDyvVZQXxq+1EaSEVHo4A2dF0OVTU2MGS7NmXS7ZQhhGkO+mMxHIu1yZJ0LmuqD8qJ2ACCtCgCp1EutOr1BqNJo25xIFvl1tt+wdTtkKfdGK9CB9frLWyDbxkIbD4UjaYAAsA6ABaciQQrjxhqhkiEcAJgAzAuFwTR6PiOAIDJ8oxXGBkXqqAByWUQA1QfO+dyuH0QGhQJAIVwoGgl+XQJvASLyu9uBgbyoCBomkZhgAQRgPG-bIIzwJkWWYOgEBQZErDKVBQTAGY+QFKAAD0AH5pBEAAiTBSMiUikEoqBSPyUiCUwBgEJ-EiAEZIgXSIlyYljYy6LtVl+cw7RsJFtRHAB1S0pnI0jLkEu0RNZN4bGAOAAAVmHyO9iioKZ2MUvBQzwIA

Live example:

https://codesandbox.io/p/sandbox/tgr8pm

💻 Use Cases

1. What do you want to use this for?
2. What shortcomings exist with current approaches?
3. What workarounds are you using in the meantime?

動機となる doubleMap の例とリンク先の TypeScript Playground から始め、ジェネリックなタプルのインデックス付きアクセスと for-of イテレーションが現在どのようにチェックされているかを調査します。入れ替えられた mapper の呼び出しが拒否され、有効なタプルおよび配列の使用では適切な要素型 T[number] または S[number] が引き続き維持されれば完了です。