[Feature Request] Support change type of this, to act on type guard of following codes

[zyf0330]

Suggestion

🔍 Search Terms

List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily and help provide feedback.

change variable type, change this type, asserts

✅ 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

We know there is is to do type guard acting on function return value. And we have as to convert type inside a statement.

Can we has a keyword to change type of variable or this as a statement, to change type and affect following all codes instead of just affect one statement like as?

📃 Motivating Example

When facingto this, I don't want to define a aliased variable like that. And use as inside every statement to convert type is cumbersome.

💻 Use Cases

It is like

function func(this: { a?: string; b: string }) {
  delete this.a
  this is {a: never; b: string} // not work now
  this.a // should not use
}

[jcalz]

Duplicate of #10421, seems to be doable via an assertion function as implemented in #32695:

function assertType<T>(a: any): asserts a is T {}

function func(this: { a?: string; b: string }) {
  delete this.a
  assertType<{a: never, b: string}>(this)
  this.a // never
}

Playground link

[zyf0330]

@jcalz Sorry, I didn't know asserts keyword. But still I just don't want to create an extra helper function to achieve what I want.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.