Comment by c-smile

This sample

    function keyByUsernameReducer(acc, person) {
      return {...acc, [person.username]: person};
    }
    const peopleObj = peopleArr.reduce(keyByUsernameReducer,   {});

has O(n*n) complexity and memory consumption of the same magnitude.

This, while being not so functional pure, is better I think:

    function keyByUsernameReducer(acc, person) {
      acc[person.username] = person;
      return acc;
    }

But still... needless function call on each iteration.