The following code in Typescript gives an assignment error on the last line
interface T {id: string}
let ts: Array<T> = []
let tMap: Map<string, T> =
new Map (
ts.map(t => [t.id, t])
)
Similar code in a javascript environment works. Cant see what I am doing wrong
The problem is that the Map constructor wants an array of two-element tuples, but TypeScript does not infer tuple types from string literals. That means, the following:
const foo = [1, "two"];
is inferred as the array type Array<string | number> and not the tuple type [number, string]. If you want it to be [number, string] you can use an explicit type annotation like
const foo: [number, string] = [1, "two"];
or a type assertion like
const foo = [1, "two"] as [number, string];
Or, if you need this a lot, since it is possible to get TypeScript to infer a tuple type from function arguments, you can make a helper function like this:
const tuple = <T extends any[]>(...t: T) => t;
and then use it:
const foo = tuple(1, "two"); // inferred as type [number, string]
Any of those will help you get [t.id, t] to behave properly. Let's try the last one:
interface T { id: string }
let ts: Array<T> = []
let tMap: Map<string, T> =
new Map(
ts.map(t => tuple(t.id, t)) // okay 👍
);
Hope that helps. Good luck!
Seems like Typescript can't infer the the map content as tuples, which should probably be reported as a bug. But I found a workaround which I think is acceptable, by explicitly writing the type of the map content:
let tMap: Map<string, T> =
new Map(ts.map<[string, T]>(t => [t.id, t]))
