what is const/type pattern in Typescript

Question

export const INCREMENT_ENTHUSIASM = 'INCREMENT_ENTHUSIASM';
export type INCREMENT_ENTHUSIASM = typeof INCREMENT_ENTHUSIASM;


export const DECREMENT_ENTHUSIASM = 'DECREMENT_ENTHUSIASM';
export type DECREMENT_ENTHUSIASM = typeof DECREMENT_ENTHUSIASM;

what is happening here? I am not able to understand this.It is very confusing This is from here https://github.com/Microsoft/TypeScript-React-Starter under Adding actions section.

I know what type keyword do but it seems very confusing here.

Answer 1

The idea behind the example you posted is to export both a value and a type so that you'll be able to do something like:

let myVar: INCREMENT_ENTHUSIASM;

If the line:

type INCREMENT_ENTHUSIASM = typeof INCREMENT_ENTHUSIASM

was missing the above variable declaration would have raised an error:

Cannot find name 'DECREMENT_ENTHUSIASM'

Some things in typescript are both a type and a value, suchs as enums and classes, but an interface or a type alias are only types, and in such cases you can then reuse the type name to create a value as well.
For example:

type MySingleton = {
    getId(): string;
    doSomething1(str: string): string;
    doSomething2(num: number): number;
}

const MySingleton: MySingleton = {
    getId: function () {
        ...
    },
    doSomething1: function(str: string): string {
        ...
    },
    doSomething2: function (num: number): number {
        ...
    }
}

More on types/values in typescript here: Declaration Merging - Basic Concepts

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Edit

type INCREMENT_ENTHUSIASM = typeof INCREMENT_ENTHUSIASM;

Is equal to:

type INCREMENT_ENTHUSIASM = "INCREMENT_ENTHUSIASM";

Which makes INCREMENT_ENTHUSIASM a string that can only be "INCREMENT_ENTHUSIASM".