Type 'X' cannot be used as an index type.

What it means

JavaScript object keys can only be strings or symbols (numbers are coerced to strings). So an index expression must be typed string, number, symbol, or a literal union of those. Anything else — an object, an array, undefined, a union that includes them — is rejected.

The most frequent real cause is a value that is nearly a valid key: string | undefined from an optional property or an array.find(). The fix is to narrow away the invalid half, not to cast.

Reproduce it

declare const key: { a: 1 };
declare const obj: Record<string, number>;
console.log(obj[key]);
//              ~~~ Type '{ a: 1; }' cannot be used as an index type.

How to fix it

const key = form.name; // string | undefined
const value = table[key];

const key = form.name;
const value = key ? table[key] : undefined;

console.log(scores[user]);

console.log(scores[user.id]);

function get(o: Config, k: string) { return o[k]; }

function get<K extends keyof Config>(o: Config, k: K) { return o[k]; }

Suppressing it

Casting the key with as string compiles but can produce undefined at runtime when the value really was missing. Narrowing costs one line and removes the risk.

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