What is a good way to find which union the variable belongs to?-CodePudding

I've been trying to create a function where the function's parameter is a union made from two other unions. I want it so that depending on the string passed into the parameter, a different function fires accordingly. Something like this:

type Type1 = "a" | "b" | "c";
type Type2 = "x" | "y" | "z";

function doSomething(param: Type1 | Type2) {
    // if param is a part of the "Type1" union
         functionForTypeOne();
}

doSomething("z");

I searched through the internet and the docs and I couldn't find a way to do this runtime type checking. I found some "solutions" for type checking, but they were all written for object types, not strings and unions.

In basic javascript, I could create a const array and check if the string is found inside the array, like so:

const type1 = ["a", "b", "c"];
const type2 = ["x", "y", "z"];

function doSomething(param) {
    if (type1.includes(param))
        functionForTypeOne();
}

Doing this in TypeScript is kind of a solution, but since I need to create unions from these consts, it feels very inefficient, and doesn't allow me to use the type checking features of the language. I was curious if anyone has a solution for this that I possibly overlooked, or if this is the only way for me to get around my problem.

CodePudding user response：

You can build the unions from your tuples almost automatically, and use a type guard predicate to recover type safety:

const type1 = ["a", "b", "c"] as const; // Assert as const to infer tuple instead of array string[]
const type2 = ["x", "y", "z"] as const;

// Automatically get union type from tuple values
type Type1 = typeof type1[number];
//   ^? "a" | "b" | "c"
type Type2 = typeof type2[number];
//   ^? "x" | "y" | "z"

// Type guard predicate
function isIn<T extends readonly string[]>(val: string, arr: T): val is T[number] {
    return arr.includes(val);
}

function doSomething(param: Type1 | Type2) {
    param
    //^? "a" | "b" | "c" | "x" | "y" | "z"
    if (isIn(param, type1)) {
        param
        //^? "a" | "b" | "c"
        //functionForTypeOne();
    }
}

Playground Link

CodePudding user response：

you may check this answer https://stackoverflow.com/a/54061487/2429434 There is no way to do what you want to do with your code.