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)

Inizia riproducendo gli esempi destrutturati e non destrutturati nel TypeScript Playground collegato. L’issue non indica alcun file sorgente né alcun test; segui come vengono gestiti i predicati di tipo e i parametri destrutturati. Il lavoro è completato quando il predicato destrutturato restringe il tipo di tupla filtrato senza modificare il JavaScript emesso, con copertura di regressione.