E0225: index signatures require a value type

A TypeScript index signature has a key type and a value type. It is a syntax error to omit the value type:

interface StringArray {
  readonly [index: number];
}

interface StringDictionary {
  [k: string];
}

To fix this error, write : then the type of the value:

interface StringArray {
  readonly [index: number]: string;
}

interface StringDictionary {
  [k: string]: any;
}

Introduced in quick-lint-js version 2.6.0.

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