Support preserving type narrowing inside closures in some simple cases #55528

🔍 Search Terms

• narrowing
• type
• closure
• function

✅ 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 feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

I understand that types are not kept narrowed inside callbacks because the narrowing could in general not hold anymore in there, however there are some "simple" scenarios where we can be sure that the narrowing holds, for example:

function foo () {

  let maybe: string | null;

  maybe = 'foo';

  console.log ( () => {

    maybe;
    // ^?

  });  

}

This scenario is a sort of a simple edge case of the general problem because:

• The maybe variable holds a primitive value, so it will be passed to other functions by copy, so those functions outside the closure can't change its value inside the closure.
• The code executed before the console.log call is done forever, since it doesn't have any callbacks that assign to maybe, and it's not wrapped in an unterminated loop.
• The "code" after the console.log call also has no assignments to "maybe", in fact it has no references to "maybe" at all.

Under these conditions it's impossible for narrowing the type inside our function in the example above to be wrong, so it should be preserved.

📃 Motivating Example

Playground: https://www.typescriptlang.org/play?#code/FAMwrgdgxgLglgewgAhAhyAUBKZBvYYZZAGwFMZkBbAQwE8AjMgLmQGcYAnOCAc2QA+yCGBIkA3IWK1GZZAF5kAcjQIlkosihI2CcgDoSCfpiy55APnxTi1ek0m3kAemfIAegH4bAX2zjiQh9gIA

💻 Use Cases

Preserving narrowing in simple cases like the one mentioned above is very handy and allows the user to write clean code in more scenarios.