Converting from string to custom type

Question

I'm very new to JS. I created a custom type called "Stage". I would like to convert strings to these types. The strings will be only ever equal one of these values exactly (first letter capitalized, written the same). I've been trying to accomplish something using .find() but this doesn't appear to be the route to go as it takes in an array. Any help is appreciated!

type Stage = 'First' | 'Second';

There are no custom types in JavaScript. Perhaps larger chunk of code would shed more light on what you need. — The Witness, Jul 12 '19 at 14:19
So I'm trying to take create a function to take in a string and convert it to "Stage": const stringToStage(str: string, stage: Stage) => — turkr, Jul 12 '19 at 14:21
JavaScript doesn't have types like that. Are you using Typescript? — Cerbrus, Jul 12 '19 at 14:21
ok typescript! That helps a lot! this is a project I'm working on in react — turkr, Jul 12 '19 at 14:22
Okay, so you have an `any` variable that you want to cast to that `Stage` type? — Cerbrus, Jul 12 '19 at 14:22
Possible duplicate of [TypeScript or JavaScript type casting](https://stackoverflow.com/questions/13204759/typescript-or-javascript-type-casting) — Cerbrus, Jul 12 '19 at 14:24
Or: [Learning TypeScript - Casting Types](https://stackoverflow.com/questions/44967356/learning-typescript-casting-types) — Cerbrus, Jul 12 '19 at 14:24
No problem. Please keep in mind that these type assertions don't throw any errors if the actual runtime value doesn't match the type. It just the compiler to assume the variable is of a certain type. — Cerbrus, Jul 12 '19 at 14:27

Burt_Harris · Answer 1 · 2019-07-15T05:52:23.147

In TypeScript, the type Stage... statement is a bit confusing because it creates a type alias rather than an actual type. Converting a string into Stage at runtime is not needed because, at runtime, a Stage is a string.

A TypeScript enum may be closer to what you are looking for. It creates an object which can be used to convert to/from strings. Try something like this test/demo routine:

import test from 'ava';

enum Stage {
    First = 1,
    Second = 2
}

test('Testing Stage conversion between string and number ', t=> {
    const stringName = "First";
    t.is(Stage.First, 1);
    t.is(Stage[stringName], 1);
    t.is(Stage.Second, 2);
    t.is(Stage[1], "First");
    t.is(Stage[2], "Second");
    t.is(Stage["foo"], undefined);
    t.pass();
})

Converting from string to custom type

1 Answers1