TS2411

error TS2411: Property ‘age’ of type ‘number’ is not assignable to ‘string’ index type ‘string’.

Broken Code ❌

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interface Person {
  [key: string]: string;
  age: number;
  name: string;
}

Fixed Code ✔️

We defined an interface where every key has a value of type string. This doesn’t work for age which is why we have to extend the possible value types using a union type:

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interface Person {
  [key: string]: number | string;
  age: number;
  name: string;
}