CODEWITHSUNDEEP
typescript
Jacob Weisenburger
Jacob Weisenburger

Reputation: 461

Function type not causing errors when I expect errors

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?

Upvotes: 1

Views: 42

Answers (1)

Matthieu Riegler
Matthieu Riegler

Reputation: 54629

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.

Upvotes: 1

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