How to convert runtime string to string enum

Question

I have a typescript enum myEnum (with string values) and a function foo returning a string. I need to convert return value of foo to myEnum and default on some value if key is not present in enum.

Let's condider this piece of code:

enum  myEnum {
    none = "none",
    a = "a",
    b = "b"
}

function foo(): string {
    return "random-string";
}

function bar(): myEnum {
    const t = foo();
    if(t in myEnum) {
        return myEnum[t];
    }
    return myEnum.none;
}

console.log(bar());

That fails with typescript 4.1.2 with error

Element implicitly has an 'any' type because expression of type 'string' can't be used to index > type 'typeof myEnum'. No index signature with a parameter of type 'string' was found on type 'typeof myEnum'

How should I do that without losing typesafety ?

Does this answer your question? How do I convert a string to enum in TypeScript? — CodeCaster
– CodeCaster, Commented Dec 7, 2020 at 14:20

spender · Accepted Answer · 2020-12-07 14:19:11Z

1

t is a string, in no-way related to the set of values in your enum.

Inherently you need to lose some type-safety, because the string could be any value. As such, you must cast it to a myEnum key for the win:

const t = foo() as keyof typeof myEnum;

answered Dec 7, 2020 at 14:19

spender

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1 Comment

Regis Portalez Over a year ago

great ! Typescript has quite evolved since version 0.8 apparently :)

ABOS · Accepted Answer · 2020-12-07 14:39:58Z

0

You can try to suppress this warning using type guard

enum  myEnum {
    none = "none",
    a = "a",
    b = "b"
}

function foo(): string {
    return "random-string";
}

function isMyEnum(s: string): s is myEnum {
    return s in myEnum;
}

function bar(): myEnum {
    const t = foo();
    if(isMyEnum(t)) {
        return myEnum[t];
    }
    return myEnum.none;
}

console.log(bar());

answered Dec 7, 2020 at 14:39

ABOS

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