Add type predicate to Object.is

[KSXGitHub]

Search Terms

object.is

type predicate

Suggestion

When a is b, a and b has to same type, so I suggest adding a few generic overloads with type predicates

interface ObjectConstructor {
  is<A, B extends A> (a: A, b: B): a is B
  is<A extends B, B> (a: A, b: B): b is A
  is (a: any, b: any): boolean
}

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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[edemaine]

I ran into this issue in realizing that Object.is(a, b) doesn't narrow types in the way that a === b does.

It seems that the new const type parameters of TS 5.0 are superior for type narrowing:

is: (<const T, U extends T>(a: T, b: U) => a is U) &
    (<const T, U extends T>(a: U, b: T) => b is U)

See playground example.

For comparison, the original poster's definition doesn't handle the second case properly.

Perhaps TS5 is the right time to add this better type for Object.is?

[edemaine]

On further reflection, we realize that these definitions don't properly handle the case that a and b have partially overlapping types.

A reasonable heuristic for now seems to be this:

const is: {
  <B, A extends B> (a: A, b: B): b is A,
  <A, B> (a: A, b: B): a is A & B
} = Object.is as any;

• If the second argument is more general, narrow it.
• Otherwise, narrow the first argument in all cases.

Playground example

Until #26916 this seems to be the best we can do.