Typeguards with destructuring parameters

Search Terms

Typeguard
type narrowing
destructuring

Suggestion

It'd be nice to be able to have typeguard functions with destructuring arguments.

Use Cases

When manipulating tuples or other anonymous structured types, whose typeguard relies only on a small part of the object, it would make the code much easier to read.
Currently, typeguards cannot be used with destructuring arguments, which makes it a weird special case.

Examples

const val: Array<[string, boolean]> = ([] as Array<[string | undefined, boolean]>)
    .filter(([foo, bar]: [string | undefined, boolean]): [foo, bar] is [string, boolean] => foo !== undefined);

const val2: Array<[string, boolean]> = ([] as Array<[string | undefined, boolean]>)
    .filter((val: [string | undefined, boolean]): val is [string, boolean] => val[0] !== undefined);

https://www.typescriptlang.org/play/index.html#src=const%20val%3A%20Array%3C%5Bstring%2C%20boolean%5D%3E%20%3D%20(%5B%5D%20as%20Array%3C%5Bstring%20%7C%20undefined%2C%20boolean%5D%3E)%0D%0A%20%20%20%20.filter((%5Bfoo%2C%20bar%5D%3A%20%5Bstring%20%7C%20undefined%2C%20boolean%5D)%3A%20%5Bfoo%2C%20bar%5D%20is%20%5Bstring%2C%20boolean%5D%20%3D%3E%20foo%20!%3D%3D%20undefined)%3B%0D%0A%0D%0Aconst%20val2%3A%20Array%3C%5Bstring%2C%20boolean%5D%3E%20%3D%20(%5B%5D%20as%20Array%3C%5Bstring%20%7C%20undefined%2C%20boolean%5D%3E)%0D%0A%20%20%20%20.filter((val%3A%20%5Bstring%20%7C%20undefined%2C%20boolean%5D)%3A%20val%20is%20%5Bstring%2C%20boolean%5D%20%3D%3E%20val%5B0%5D%20!%3D%3D%20undefined)%3B%0D%0A%0D%0Aconst%20val3%3A%20string%5B%5D%20%3D%20(%5B%5D%20as%20Array%3Cstring%20%7C%20undefined%3E)%0D%0A%20%20%20%20.filter((foo%3F%3A%20string)%20%3D%3E%20foo%20!%3D%3D%20undefined)%3B%0D%0A

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. new expression-level syntax)

リンクされた TypeScript Playground で、分割代入ありと分割代入なしの例を再現することから始めてください。issue にはソースファイルもテストも指定されていません。型述語と分割代入されたパラメーターがどのように扱われるかを追跡してください。分割代入された述語によってフィルター処理されたタプル型が絞り込まれ、出力される JavaScript が変更されず、回帰テストのカバレッジが追加されれば完了です。