title: array reduce accumulator as an empty array, push not working
body: <p>Why is the accumulator.push  not valid? accumulator is an empty array!? If i use the predefined variable <code>flarArray</code>, things work just fine!! I know that if no initial value is included in the reduce callback, then it will use the first item in the array (which in my case will be an object) to be the accumulator, but if I tell it to start off as an array as the accumulator, what is the problem with that?</p>
  
  <pre><code>const profiles = [
    {
      id: 1,
      userID: '1',
      favoriteMovieID: '1',
    },
    {
      id: 2,
      userID: '2',
      favoriteMovieID: '1',
    }
  ];
  
  const users = {
    1: {
      id: 1,
      name: 'Jane Cruz',
      userName: 'coder',
    },
    2: {
      id: 2,
      name: 'Matthew Johnson',
      userName: 'mpage',
    }
  };
  
  const movies = {
    1: {
      id: 1,
      name: 'Planet Earth 1',
    },
    2: {
      id: 2,
      name: 'Selma',
    }
  };
  let flatArray = [];
  const flattenedProfiles = profiles.reduce(function(accumulator, currentProfile) {  
    return accumulator.push({id: currentProfile.id, name: users[currentProfile.id].name, favoriteMovie: movies[currentProfile.favoriteMovieID].name})
  },[]);
  </code></pre>