Object.groupBy types are wrong (remove Partial<>)

[fregante]

This isn't possible. Each key of grouped and its value must exist:

const grouped = Object.groupBy(
  [
      {
          name: "foo",
          value: 20,
      },
      {
          name: "bar",
          value: 60,
      },
      {
          name: "bar",
          value: 40,
      }
  ],
  ({name}) => name,
);
for (const group of Object.values(grouped)) {
  group.forEach(console.log)
  // 'group' is possibly 'undefined'.(18048)
}

https://www.typescriptlang.org/play/?checkJs=true&allowJs=true&target=99&types=%5B%22node%22%5D#code/FAAhGMHsDsGcBcQHMBOkCuAHApgExALwgDyARgFbbjwB0qGmAQgJ4AUoYIA2h55wN68+w6AEMAttgBcIAEQAzSJFkAaIcM4A3UQBt00kACYADGo1gAvmfODzIiQdmlRKVevPa9BgGyn3l6w1bO04xSRknFzcQrV19GQAWPzsLIQBdQJBWfjDsCwBKQgA+EFzrfIBuDkUULKg4RHosEEh5EgoqWk99WFYmnFx8wuCwfpoagFFRcAALVnrYSB1sGh1IJHyhAHotkAByfr2QAEtYEExIWFhj0h1mffRoXGx5Y+g8PZpWAEYADmMEr9NpYgA

Additional comments may be found on microsoft/TypeScript#47171 (comment)