How to add to array?

Question

JavaScript

In general, how to do so, to add to the array? I keep getting an error.

class Live {
  constructor()
  {
    this.users = [];
  }
  add(user)
  {
    if(typeof user.play !== 'undefined')
    {
      this.users.push({
        username : user.username,
        play : user.play,
      });
    }
  }
  get()
  {
    return this.users;
  }
}

When you call add(user) , you need to add an entry to the users array, but an error occurs. How to fix ?
I used to work in PHP, it works like that)