Type '{ [x: string]: number; }' is not assignable to type '{ [K in Key]: 123; }'

function hello<Key extends string>(key: Key): { [K in Key]: 123 } {
  return ({ [key]: 123 }); // as { [K in Key]: 123 }
}

hello<'myKey'>('myKey');

I don’t get it why the code snippet above produces an error and key is treated as string in [key] instead of Key.

How can I solve it? I commented out a workaround but it just feels wrong.

>Solution :

This is a pretty common issue with the dynamic key. This can be fixed by adding a separate function that will be responsible purely for this operation:

function put<Key extends string, Value>(
  key: Key,
  value: Value,
): { [K in Key]: Value };
function put<Key extends string, Value>(key: Key, value: Value) {
  return { [key]: value };
}

Testing:

function hello<Key extends string>(key: Key): { [K in Key]: 123 } {
  return put(key, 123); // no error
}

hello('myKey'); // no error

playground