Modify an Array Stored in an Obj

Why i cannot return right away in a single line as i did here, and not repeated?
Answer isshowing:
userObj.data.friends.push(friend);
return userObj.data.friends;


let user = {
name: 'Kenneth',
age: 28,
data: {
  username: 'kennethCodesAllDay',
  joinDate: 'March 26, 2016',
  organization: 'freeCodeCamp',
  friends: [
    'Sam',
    'Kira',
    'Tomo'
  ],
  location: {
    city: 'San Francisco',
    state: 'CA',
    country: 'USA'
  }
}
};

function addFriend(userObj, friend) {
// Only change code below this line
return userObj.data.friends.push(friend);

// Only change code above this line
}

console.log(addFriend(user, 'Pete'));
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Challenge: Modify an Array Stored in an Object

Link to the challenge:

What does the push method return?

I always forget all the details of all these methods. I just look them up.