I think you are running into a common TypeScript issue. When you use keyof Foo, the value type associated with the key won't might not match what you are assigning. Since Foo, has a mix of required and optional properties with different types, TypeScript can't infre a safe assignment type for existingProduct[key] = newProduct[key].

You can fix this like so:

interface Foo {
  id: string;
  bar?: number;
  }
 
function setIfChange<k extends keyof Foo> (
  newProduct: Foo,
  existingProduct: Foo,
  key: k
): boolean {
  if (newProduct[key] !== existingProduct[key]) {
     existingProduct[key] = newProduct[key];
     console.log(`Product ${key} updated:`, 
     newProduct);
     return true;
   }
   return false;
}

basically, by adding , Typescript now knows that key is a specific key of Foo, and newProduct[key] and existingProduct[key] are both of type Foo[k]

This helps to avoid type assertion and // @ts-ignore while keeping type safety.