TypeScript - Element implicitly has an 'any' type because expression of type 'string' can't be used to index type

You can declare colors as any to tell TypeScript to get off your back on this one (aka explicit any):

const color : any = {
    red: null,
    green: null,
    blue: null
};

But if possible, strong typing is preferable:

const color : { [key: string]: any } = {
    red: null,
    green: null,
    blue: null
};

More information on indexing in TypeScript: Index Signatures

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EDIT: In this answer to a similar question, the author suggest using a Map<,> -- if that fits your use-case.

The problem you're having is not that color is the wrong type, but that TypeScript is inferring the type of ['red', 'green', 'blue'] to be string[]. Often that type of inference is desirable, since (for all the compiler knows) you might want to push 'purple' onto it. But in this case, you'd like the compiler to know that the only members are the three string literals 'red', 'green', and 'blue'. That is, you need a type at least as specific as Array<'red'|'green'|'blue'>.

Assuming you're using TS3.4 or later, the easiest way to get this kind of type inference from the compiler is to use a const assertion:

const constAssertionTest = ["red", "green", "blue"] as const;
// const constAssertionTest: readonly ["red", "green", "blue"];

The as const causes the compiler to infer a tuple composed of exactly the three string literals in the array, in the exact order you've set. (It's even a read-only tuple). That is good enough to fix your error:

const newColor = (['red', 'green', 'blue'] as const).filter(e => color[e]); // okay

All right, hope that helps. Good luck!

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