Propagating information from type guards to parent objects

[fabiospampinato]

🔍 Search Terms

• narrowing
• parent
• propagate

✅ 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

An example is worth a 1000 words:

type Process = undefined | {
  exitCode: number | null
}

const isNull = ( value: unknown ): value is null {
  return value === null;
};

let process: Process;

if ( isNull ( process?.exitCode ) ) {
  process.exitCode
  // ^?
}

We are narrowing process.exitCode, but indirectly we should be also narrowing the parent objects, which in this case is just process. Basically if process.exitCode is null then process can't be undefined, still, TS thinks it might still be.

📃 Motivating Example

See above.

💻 Use Cases

Generally the smarter the type checker the better, and the cleaner the code you can write gets.

[MartinJohns]

Duplicate of #42384.

[fabiospampinato]

Aaaaah, missed that, thanks!

[fatcerberus]

@fabiospampinato Note that this form of narrowing (narrowing a parent object to be non-null off the result of an optional chain) actually already works when done inline:

let process: Process;

if (process?.exitCode === null) {
  process.exitCode  // ok
}

Playground link

Not sure why it doesn't work with UDTGs. I guess technically a type predicate can lie but the compiler generally assumes they don't.