Utilizing the Literal Form of a Generic Object Parameter

I'm looking for a way to modify the return type of a function that accepts a generic object. I want the return type to be identical to the object passed in, but narrowed down similar to how `as const` assertions work.

For example, if I call the function with `{ a: "first", b: "second" }`, I want the return type to be `{ a: "first"; b: "second" }` instead of just `{ a: string; b: string }`:

myFn({ a: "first", b: "second" });

Is there a way to instruct the type-checker to narrow down the return type of `myFn` based on its first argument?

typescriptgenericscastingconstantsnarrowing

Answer №1

Ensure that the return type matches the object, but with narrowed constraints similar to how const assertions function.

To achieve this, simply pass the type as a parameter to the function.

function customFunction<T>(args): T {
  // perform actions
  return null;
}

const result = customFunction<{ key: "value"; anotherKey: "anotherValue" }>(args);


type MyObject = { key: "value", anotherKey: "anotherValue" }
const result = customFunction<MyObj>(args);

Keep in mind that the object must be fully defined at compile time since types aren't accessible during runtime.

Prior answer (initial misunderstanding)

If you wish for the return type to mirror the argument type, utilize the generic type parameter.

function OtherFunction<T>(arg1: T, otherArg: number): T {
  // carry out operations
}

// final result will be of type User
const output = OtherFunction<User>(user, 9);

In certain scenarios, you can simply:

// user should have been previously defined as type User
const output = OtherFunction(user, 9);

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