Tagged unions and tagging using literal types

Question

In TypeScript 2.0 there were introduced tagged unions. To use them we have to introduce a discriminant property in interface, for example:

interface Action {
    type: "ACTION"
}

However I'm not able to use string literal type as a discriminant:

let actionName: "ACTION"

interface Action {
    type: actionName <- error: cannot find name "actionName"
}

I'm wondering if it's a feature or a bug.

Answer 1

Your string literal definition is wrong. let defines variable, not a type. It should be:

type actionName = "ACTION"

interface Action {
    type: actionName;
}

Answer 2

The solution is to use the following notation:

const ACTION: "ACTION" = "ACTION"

interface Action {
    type: typeof ACTION
}