String literal not assignable to type of the same value, forcing me to cast

[atrauzzi]

I'm not sure if this has been brought up already and I'm just not plugging in the right search terms, but I feel like the compiler is forcing me to do a possibly redundant type annotation with string literal types.

TypeScript Version: 2.3.3

export interface AddAuthor extends Action {

    type: "add-author";

    author: Author;
}

export interface BusReducer<ActionType extends Action, State = any> {

    action: ActionType["type"];

    reducer(state: State, action: ActionType): State;
}

export class AddAuthor implements BusReducer<Action> {

    // tsc doesn't like this, failing with "Type 'string' is not assignable to type '"add-author"'."
    public action = "add-author";

    // But this seems to work...
    public action: "add-author" = "add-author";

    // ...
}

Expected behavior: Basically I expect that action in my BusReducer subtype not require its type be explicitly declared as the interface contains everything the lookup should need to successfully match on the constant value.

[DanielRosenwasser]

Duplicate of #10570. I do appreciate that you tried to look it up. It wasn't easy for me to dig up either. 😄

[typeofweb]

I have a very similar problem, although not related to implementing Interfaces, just to compatibility between string literal type and string literal values. See this very simple example:

const data = {
    myLiteral: 'yas'
};

type MyType = {
  myLiteral: 'yas' | 'nope';
};

function doSomething(productData: MyType): void {}

doSomething(data); // error Type 'string' is not assignable to type '"yas" | "nope"'.

https://www.typescriptlang.org/play/index.html#src=const%20data%20%3D%20%7B%0A%20%20%20%20myLiteral%3A%20'yas'%0A%7D%3B%0A%0Atype%20MyType%20%3D%20%7B%0A%20%20myLiteral%3A%20'yas'%20%7C%20'nope'%3B%0A%7D%3B%0A%0Afunction%20doSomething(productData%3A%20MyType)%3A%20void%20%7B%7D%0A%0AdoSomething(data)%3B%0A

[kitsonk]

const objects are not inferred to their literal type, because objects properties are mutable. The const only prevents re-assignment. There are slightly different rules when specifying an object literal as an argument, because it favours usability over strict typing. So while doSomething(data) throws, the following does not:

type MyType = {
  myLiteral: 'yas' | 'nope';
};

function doSomething(productData: MyType): void {}

doSomething({
  myLiteral: 'yas'
});

This is because TypeScript can easily statically determine that the myLiteral property won't be changing. Of course, data is easily assignable to MyType. This also works:

const data: MyType = {
    myLiteral: 'yas'
};

type MyType = {
  myLiteral: 'yas' | 'nope';
};

function doSomething(productData: MyType): void {}

doSomething(data);

These sort of general questions are better asked on StackOverflow or in Gitter.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

[atrauzzi]

Unfortunately the referenced duplicate seems to have gone dead. Any chance it can get attention?