Inline condition type narrow

[behnammodi]

Suggestion

🔍 Search Terms

Inline condition type narrow

✅ 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

Well, if we have type narrow front of condition this would be very helpful

if (typeof thing.x === 'number'): thing is A {
  ...

📃 Motivating Example

This is very helpful to reduce redundant code for type narrow, without that we have to write a function to check type:

...
const isA = (thing: A | b): thing is A => typeof thing.x === 'number';

if (isA(thing)) {
    console.log(thing.x + 1);
}
...

💻 Use Cases

class A {
    x: number = 0;
}

class B {
    x: string = ""; 
}

const thing: A | B = {} as any;

if (typeof thing.x === 'number'): thing is A {
    console.log(thing.x + 1);
}

[MartinJohns]

Duplicate of #41710, which was marked a duplicate of #6474.

[fatcerberus]

For the record, this is no safer than just using a type assertion, since the type predicate isn’t checked (if it could be checked, you wouldn’t need it). The same goes for type guard functions, but those at least are reusable.

[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.