JS Error: Cannot read property 'length'

I want to get a random number from an array but I am getting the error: Cannot read property 'length'.

  //Attack arr
  var attackID = [0,1];

  //Start game
  function start(){
    console.log("Your new username is "+localuser.username+"!");
    var r_atk = Math.round(Math.random()*(attackID.length-1));

    if(attacks[r_atk] == 0){
      punch(enemy.name);
    }
    else if(attacks[r_atk] == 1){
      kick(enemy.name);
    }
  }

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