I have the following types defined
type MyUnionType = "propA" | "propB"
type MyType = {
myProp: MyUnionType
}
type SomeOtherType = MyType & { ... /* other props*/ ... }
Now, I would like to enhance MyType with something along the lines of
type MyType = {
myProp: MyUnionType,
[key in MyUnionType]: any // pseudo code
}
so that I can use it like this
const obj: SomeOtherType = {
myProp: "propA",
propA: "some value" // if this is missing, compiler will error
[...] // some other props
}
I played around it a little bit, searched the web, but I didn't come up with any solution.
EDIT: I just removed string from MyUnionType as requested
For this to work, you want MyType to be a union type equivalent to:
type MyType = {myProp: "propA", propA: any} | {myProp: "probB", propB: any}
This has the following desirable behavior:
const goodObjA: MyType = {
myProp: "propA",
propA: "some value"
} // okay
const badObjA: MyType = { // error! Property propA is missing
myProp: "propA",
}
const goodObjB: MyType = {
myProp: "propB",
propB: "some value"
} // okay
const badObjB: MyType = {
myProp: "propB",
propA: "oops" // error, unexpected property
}
So the only question now is how to generate MyType programmatically from MyUnionType. Conceptually we want to take each member K in the MyUnionType union, and produce a new union of { myProp: K } & { [P in K]: any } (which has a myProp property of type K, and a K-keyed property of type any. That is, we want to distribute the type F<K> = { myProp: K} & { [P in K]: any } operation across the MyUnionType union.
This can be done a few ways, but my approach is to make what's called a distributive object type as coined in microsoft/TypeScript#47109. The idea is that you make a mapped type over each K in MyUnionType, and then immediately index into it with MyUnionType, producing the desired union:
type MyType = { [K in MyUnionType]:
{ myProp: K } & { [P in K]: any }
}[MyUnionType]
This evaluates to
type MyType =
({ myProp: "propA"; } & { propA: any; }) |
({ myProp: "propB"; } & { propB: any; });
which is equivalent to the type we want.