Why do TypeScript interfaces not support index signatures while type aliases do? [duplicate]

Question

I have a type with a string index signature:

declare var result: {
  [key: string]: number;
};

Assigning an interface to the type fails:

interface IData {
  a: number;
  b: number;
}
declare var data: IData;
result = data; // Error: Type 'IData' is not assignable to type '{ [key: string]: number; }'. Index signature for type 'string' is missing in type 'IData'.

But assigning a type alias succeeds:

type TData = {
  a: number;
  b: number;
};
declare var data1: TData;
result = data1; // [SUCCEED] why?

I expected interfaces and type alias to have the same behavior when assigning to a type with an index signature. Why do they actually behave differently?

TS Playground link

Paul Han · Accepted Answer · 2019-10-16 08:52:48Z

2

redundant, subscribe to this issue, and the spec is out of date

https://github.com/microsoft/TypeScript/issues/15300

answered Oct 16, 2019 at 8:52

Paul Han

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