How to make Typescript infer the keys of an object but define type of its value?

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So you want something that infers keys but restricts the value types and uses excess property checking to disallow extra properties. I think the easiest way to get that behavior is to introduce a helper function:

// Let's give a name to this type
interface ElementType {
  nodes: number,
  symmetric?: boolean
}

// helper function which infers keys and restricts values to ElementType
const asElementTypes = <T>(et: { [K in keyof T]: ElementType }) => et;

This helper function infers the type T from the mapped type of et. Now you can use it like this:

const elementsTyped = asElementTypes({
  square: { nodes: 4, symmetric: true },
  triangle: { nodes: 3 },
  line: { nodes: 2, notSymmetric: false /* error where you want it */} 
});

The type of the resulting elementsTyped will (once you fix the error) have inferred keys square, triangle, and line, with values ElementType.

Hope that works for you. Good luck!

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