TypeScript TS7015 error when accessing an enum using a string type parameter

If you're using TypeScript 2.1+, you can change enumKey's type to keyof typeof State, like this:

function Emote(enumKey: keyof typeof State) {...}

or, if the function's input is required to be a string, this:

var state : State = State[enumKey as keyof typeof State];

Explanation:

Because enumKey is an arbitrary string, TypeScript doesn't know whether enumKey is the name of a member of State, so it generates an error. TypeScript 2.1 introduced the keyof operator which returns a union of the known, public property names of a type. Using keyof allows us to assert that the property is indeed in the target object.

However, when you create an enum, TypeScript actually produces both a type (which is typically a subtype of number) and a value (the enum object that you can reference in expressions). When you write keyof State, you're actually going to get a union of the literal property names of number. To instead get the property names of the enum object, you can use keyof typeof State.

Sources:

https://github.com/Microsoft/TypeScript/issues/13775#issuecomment-276381229 https://www.typescriptlang.org/docs/handbook/enums.html#enums-at-compile-time

I suspect it has to do with TS 1.8.x's new support for string literals in these situations. TS happens to know that "Happy" is a valid string index, but it doesn't know whether enumKey will be or not. You can fix it by casting it to an <any>, like so:

function Emote(enumKey:string) {
    console.log(State[enumKey]); // error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.
    console.log(State["Melancholy"]); // error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.
    console.log(State["Happy"]); // no error
    console.log(State[<any>enumKey]); // no error
    console.log(State[<any>"Melancholy"]); // no error
}

(BTW, I think this is new: I couldn't reproduce this error with 1.8.9, but as soon as I upgraded to 1.8.10, I could.)

Also interestingly, I would have expected this to work without the error, but it doesn't:

function TypedEmote(enumKey:'Happy'|'Sad'|'Drunk'){
    console.log(State[enumKey]);
}

Must be something about the TS spec I don't understand, or perhaps they just haven't gotten around to fixing that bit yet.

You can prevent this error with the compiler option without loosing the whole strict null checks

"suppressImplicitAnyIndexErrors": true

https://www.typescriptlang.org/tsconfig#suppressImplicitAnyIndexErrors

var stateName = "Happy"
var state = <State>parseInt(State[<any>stateName]);

This is what I had to do to make the compiler happy