Type that adds a property

[mightyiam]

Would this type be welcome?

Similar to Merge, but for a single property.

Not certain regarding the name.

export type Add<T, V, N extends string | symbol> = T & { [P in N]: V }

@gomain

[quezak]

@mightyiam is there a difference why you can't use Merge directly?

// also, you say "for a single property" but your example can add multiple properties, was this intended? For example Add<{}, number, 'a' | 'b' | 'c'> or even Add<{}, number, string>

[gomain]

It's for a usecase where N is inferred from a value

const AddProperty = <T, V, N extends string>(object: T, value: V, name: N): Add<T,V,N> = {
  return {
    ...object,
    [name]: value
  }
}

[quezak]

You could still write the above example's return type as Merge<T, { [k in N]: V }>.

@krzkaczor what do you think, is it worth sucha a "convenience alias"? I'm not 100% sure, because this really is just a different way to call Merge in a specific case where all the new properties have the same type. But also I'm not against it 😃 though I'd reorder the args to <T, KeyT, ValueT> since it's more intuitive

[mightyiam]

It's meant for a single property. Does anyone happen to know how that could be implemented?

[quezak]

It's meant for a single property. Does anyone happen to know how that could be implemented?

Exactly the same, only with some more complex requirements on what N can be (in particular that it can't have multiple values, for example using #100). But I don't think that's a problem, since your proposal actually works correctly when N / KeyT includes multiple properties.

[papb]

Hello, I think the following would give a better intellisense when hovering on VSCode:

export type Add<T, V, N extends string | symbol> = { [P in keyof T | N]: P extends N ? T[P] & V : T[P] }

[gomain]

Hello, I think the following would give a better intellisense when hovering on VSCode:

export type Add<T, V, N extends string | symbol> = { [P in keyof T | N]: P extends N ? T[P] & V : T[P] }

Shouldn't it be

type Add<T, V, N extends string | symbol> = { [P in keyof T | N]: P extends N ? V : T[P] }

[papb]

@gomain The type proposed by OP was

type Add<T, V, N extends string | symbol> = T & { [P in N]: V }

Therefore to be equivalent, it should be the way I proposed it.

[krzkaczor]

Thanks for the proposal but I think it's better to keep API smaller with only Merge type. We are already having problems with the number of types in this lib.