Give then following:
interface Datum {
[k: string]: any
}
type Data = Datum[]
const inner = (data: Data) => {
console.log(data)
};
const outer = (data: Data | Data[]) => {
inner(data) // expect type error?
};
I dont understand how I dont get a type error on the inner function call. I guess it has something to do with the generic. Cant figure out a way of rewriting.
The Datum interface is just too broad. It will represent basically any object (ie indexable by a string and the result of the indexing operation is any).
Given the object type is soo wide, both Datum and Datum[] and Datum[][] are assignable to an array type, and thus you don't get any error. Make the interface a bit more specific and you get an error. Ex:
interface Datum {
[k: string]: boolean | number | string;
}
type Data = Datum[]
const inner = (data: Data) => {
console.log(data)
};
const outer = (data: Data | Data[]) => {
inner(data) // error now
};
Or as @PatrickRoberts mentions in comments you can add a number index to the interface to make it explicitly incompatible with an array:
interface Datum {
[i: number]: never
[k: string]: any;
}
type Data = Datum[]
const inner = (data: Data) => {
console.log(data)
};
const outer = (data: Data | Data[]) => {
inner(data) // error
};
[i: number]: never to Datum instead of modifying the string keyed signature.
The best option would probably be using directly the object interface instead of Datum:
interface Datum {
[k: string]: any
};
// This is an object interface.
type Data = object[]; // instead of Datum[]
const inner = (data: object[]) => {
console.log(data)
};
Pd: I was looking for a solution for this same issue, used the solution from @titian-cernicova-dragomir which worked nicely. I ended up finding the object interface being used in the react-csv library.
type Data = Datum[]