Constrained generic type parameter with default value

[ember stump]

https://www.typescriptlang.org/play?#code/PTBQIAgQQhjB7AdgZwC4CcCGBLRBTAEwgHM991tYJUBPABzwjsywFs9U90IB3bVABYQCeAGaYArgBtUwzKkwA6cGFETEsVNiQRRAHgAi8zBDwAPTogLIIAJTwJ0BPZkQ0ANBFc0AfBAC8EADeAL4+ABQExgBcEEYKAcEhAJTBoBAQ6BwS6IhyCgDcoCFAA

How to make this work? I want to constrain the generic type parameter but also have a default value of an empty object to data.

[stark summit]

@ember stump Here's a shortened URL of your playground link! You can remove the full link from your message.

Fernando-Basso#5803

Playground Link

Preview:ts // // A constrained generic type parameter with default data. // function f<Data extends Record<any, any> = {}>( data: Data = {} ) { return data }

You can choose specific lines to embed by selecting them before copying the link.

[ember stump]

One solution seems to do this:

function f<Data extends Record<any, any> = {}>(
  data: Data | {} = {}
) {
  return data;
}

But maybe the are other, more appropriate ways?

[stark summit]

Ascor8522#7606

Playground Link

Preview:ts // // A constrained generic type parameter with default data. // function f<Data extends Record<any, any>>( data: Data = {} as Data ) { return data }

You can choose specific lines to embed by selecting them before copying the link.

[sharp ibex]

!ts

[stark summit]

//
// A constrained generic type parameter with default data.
//
function f<Data extends Record<any, any>>(data: Data = {} as Data) {
  return data;
}

[sharp ibex]

but that solution isn't perfacet either, since the consumer can provide a generic and no value

and there is very likely the default value doesn't match the generic

there is a snippet about that iirc

[ember stump]

Yeah, I also considered using type assertions.

Also found this earlier: https://github.com/microsoft/TypeScript/issues/49158

[river sparrow]

i think that overloads are the only way to get the types to work out the way you want:

[stark summit]

mkantor#7432

Playground Link

Preview:ts ... function f(): {} function f<Data extends Record<any, any>>( data: Data ): Data function f<Data extends Record<any, any>>( data?: Data ) { return data ?? {} }

[river sparrow]

but you have to be super careful in the implementation to uphold the promise of that second overload signature (basically you have to return data unchanged unless it's undefined)

TS won't check that for you