Feature request: Override and extend callback argument

🔍 Search Terms

extend callback argument typescript

✅ 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

(Possibly a duplicate, but I couldn't find similar issue)
I often need to change type of a callback argument and it always takes an extra line to perform as assertions. I propose to add ability to use as keyword directly on the argument.

function withCallback(callback: (arg: { foo: string }) => void) {
  // ...
}

withCallback((arg) => {
  const { foo, bar } = arg as { foo: string; bar: string }; // always an extra line
  // ...
});

📃 Motivating Example

It would be nice to have a built-in TS feature that makes possible to do that in a shorter manner:

withCallback(({ foo, bar } as { foo: string; bar: string }) => {
  console.log(foo, bar);
});

// or
withCallback(({ foo, bar } as unknown as { foo: string; bar: string }) => {
  console.log(foo, bar);
});

Overriding the type completely isn't needed too often that's why I propose to also enrich this syntax with extends (or something more applicable) keyword:

withCallback(({ foo, bar } as extends { bar: string }) => {
  console.log(foo, bar);
});

Thinking thru it I think it would make sense to also add as extends for other often appearing use-cases:

const x = { foo: 'foo' };

const y = x as extends { bar: string };
// same as 
const y = x as typeof x & { bar: string };

as extends T at this syntax can be interpreted as as typeof __SELF__ & T

💻 Use Cases

There are million use-cases for this feature. Type assertions in TS are used more often than we might expect while learning it.

The issue names no source files, tests, or entry points. Start by comparing the proposed callback-parameter and as extends syntax forms and reviewing the design discussion; the work is complete only after one scoped proposal is agreed.