SOLVED: How can I restrict a generic key using its values type?

Issue

This is not working because Item[Key] can be anything and I want to restrict key be either a string or a number. Errors are highlighted with ** and it says: Type does not satisfy the constraint string | number | symbol

function groupBy<
 Item, 
 Key extends keyof { [T in keyof Item]: Item[T] extends PropertyKey ? Item[T] : never}
>(items: Item[], key: Key): Record<**Item[Key]**, Item>{
 const groups = {} as Record<**Item[Key]**, Item>;
 for(const item of items){
   groups[item[key]] = {
     ...groups[item[key]],
     ...item
   };
 }
 return groups;
}

I also tried this but then i get this other error when I call the function, with the second parameter as 'name': Object literal may only specify known properties, and 'id' does not exist in type 'Record<"name", PropertyKey>'.

function groupBy<
  Item extends Record<Key, PropertyKey>,
  Key extends keyof Item
>(items: Item[], key: Key): Record<Item[Key], Item>{
  const groups = {} as Record<Item[Key], Item>;
  for(const item of items){
    groups[item[key]] = {
      ...groups[item[key]],
      ...item
    };
  }
  return groups;
}

Expected results:

groupBy([
  {id: 1, name: 'Marc', isAlive: false },
  {id: 1, age: 21 },
], 'isAlive'); //Should give me a type error


groupBy([
  {id: 1, name: 'Marc', isAlive: false },
  {id: 1, age: 21 },
], 'id'); //Should work

groupBy([
  {id: 1, name: 'Marc', isAlive: false },
  {id: 1, age: 21 },
], 'name'); //Should work

groupBy([
  {id: 1, name: 'Marc', isAlive: false },
  {id: 1, age: 21 },
], 'age'); //Should work

Solution

Okay I found the solution combining multiple solutions on here:

type KeysMatching<Item, Prop> = {
  [Key in keyof Item]-?: Item[Key] extends Prop ? Key : never
}[keyof Item]

function groupBy<
  Item extends Record<Key, PropertyKey>,
  Key extends KeysMatching<Item, PropertyKey>
>(items: Item[], key: Key): Record<Item[Key], Item>{
  const groups = {} as Record<Item[Key], Item>;
  for(const item of items){
    groups[item[key]] = {
      ...groups[item[key]],
      ...item
    };
  }
  return groups;
}

groupBy([
  {id: 1, name: 'Marc', isAlive: false },
  {id: 1, age: 21, isAlive: true },
], 'isAlive'); //Should give me a type error


groupBy([
  {id: 1, name: 'Marc', isAlive: false },
  {id: 1, age: 21 },
], 'id'); //Should work

groupBy([
  {id: 1, name: 'Marc', isAlive: false },
  {id: 1, name: 'Marc',age: 21 },
], 'name'); //Should work

groupBy([
  {id: 1, age: 21,name: 'Marc', isAlive: false },
  {id: 1, age: 21 },
], 'age'); //Should work

Answered By - mgm793