Proposal: add type guard keyIn

[tychenjiajun]

Usage

const a = 'foo';

const obj = {
    foo: 1
}

if (keyIn(a, obj)) {
    // a is literal type 'foo' now
}

Playground

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See microsoft/TypeScript#43284

Also, see microsoft/TypeScript#21732 I think our objectHasOwn has partially done the job.

[sindresorhus]

I think our objectHasOwn has partially done the job.

By "partially", do you mean that it only gets the "own" keys and you want any key?

Maybe we should name the utility objectHas?

[younho9]

By "partially", do you mean that it only gets the "own" keys and you want any key?

Additionally objectHasOwn guards that a "property's existence", but does not infer the "type of property".

Is there a need to improve objectHasOwn to infer the type of property as well?

[sindresorhus]

Additionally objectHasOwn guards that a "property's existence", but does not infer the "type of property".

How would it infer the "type of property" if that info not already there?

[sindresorhus]

Is there a need to improve objectHasOwn to infer the type of property as well?

I'm always happy to make any of the methods more strongly-typed whenever possible.

[younho9]

How would it infer the "type of property" if that info not already there?

It seems impossible yet. I guess he described it as "partially" for this reason.

[tychenjiajun]

Let's limit the discussion to microsoft/TypeScript#43284 , I'll open a new issue for microsoft/TypeScript#21732

[sindresorhus]

I'm ok with adding keyIn. But the docs should be clear about the differences it and the normal key in object and with objectHasOwn.