Function type not causing errors when I expect errors

Question

I'm guessing I'm not doing something quite right, but I'm not sure what it is.

I'm wanting to write a type that describes a function and it's return type.

type SomeFn<Output extends Record<string, any>> = ( x: Record<string, any> ) => Output

But when I use it, I don't get errors when I would expect them.

type ExpectedOutput = { num1: number }

const testFn1: SomeFn<ExpectedOutput> = x => ( { foo: 'foo', num1: 1 } )
//                     should error, but doesn't ^^^^^^^^^^ 

const testFn2: SomeFn<ExpectedOutput> = x => {
    const data = { foo: 'foo', num1: 1 }
    return data
    //     ^^^^ should error, but doesn't
}

What am I doing wrong?

Related: github.com/microsoft/TypeScript/pull/40311

tokland
– tokland

2022-07-29 20:28:13 +00:00
Commented Jul 29, 2022 at 20:28 — tokland
– tokland, Commented Jul 29, 2022 at 20:28

Matthieu Riegler · Accepted Answer · 2022-07-29 20:21:31Z

1

That's because of Typescript's structural typing :

Something like this is valid in TS :

type SomeFn = () => { num1: 1 }    
const testFn1: SomeFn = () => ({ num1: 1, foo: 'bar' })

What you might want is Exact types but it's an open ticket at the moment.

answered Jul 29, 2022 at 20:21

Matthieu Riegler

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