你能让 TS 从被赋值的值中推断出变量的泛型类型 arguments 吗？

Question

Answering this question the solution was to specify type arguments on the Map constructor, like this:

const conditions3: ReadonlyMap<string, any> = new Map<string, any>([
// −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−^^^^^^^^^^^^^
    [FirstName.Alex, 'foo'],
    [Lastname.Smith, 'bar']
]);

Note that the OP wanted to give conditions3 the type ReadonlyMap<string, any> , not just a Map<string, any> ; otherwise we could have just removed the type annotation from conditions3 entirely.

Unfortunately, that means repeating the type arguments on both ReadonlyMap and Map .

In the general case, is there a way to tell TypeScript to infer the type arguments for the variable/const's type ( ReadonlyMap in this example) from the type arguments on the value being assigned ( Map in this example)? I don't mean solutions to this specific case (I think I'd probably have a function, perhaps one that literally provided a read-only map, at least in development builds), but a general infer-from-the-target solution?

My various naive approaches don't work (the first two inspired by Java's <> ):

const conditions3: ReadonlyMap<> = new Map<string, any>([
//                 ^^^^^^^^^^^^^−−−−− Generic type 'ReadonlyMap<K, V>' requires
//                                    2 type argument(s).(2314)
    [FirstName.Alex, 'foo'],
    [Lastname.Smith, 'bar']
]);

playground link

const conditions3: ReadonlyMap<string, any> = new Map<>([
// Same error with no matching overloads the OP had −^^
    [FirstName.Alex, 'foo'],
    [Lastname.Smith, 'bar']
]);

playground link

const conditions3: ReadonlyMap = new Map<string, any>([
//                 ^^^^^^^^^^^−−−−− Generic type 'ReadonlyMap<K, V>' requires
//                                  2 type argument(s).(2314)
    [FirstName.Alex, 'foo'],
    [Lastname.Smith, 'bar']
]);

playground link

Is the repetition avoidable in the general case?

Here's the setup code (so it's in the question, not just linked):

export enum FirstName {
    Alex = 'alex',
    Bob = 'bob'
}
    
export enum Lastname {
    Smith = 'smith',
    Abrams = 'abrams'
} 

Answer 1

There is no syntax for partial inference in variables unfortunately.

The only work around would be to use a function as you mention in the comments:

function roMap<Key, Value>(map: Map<Key, Value>): ReadonlyMap<Key, Value> { return map; } 

const conditions3 = roMap(new Map<string, any>([
    [FirstName.Alex, 'foo'],
    [Lastname.Smith, 'bar']
]));

Playground Link