var Enemy = function(game, key , enemyHp) {
  //Call constructor of Phaser.Sprite to initialize this
  Phaser.Sprite.call(this, game, 0, 0, key );
  //Set the origin point of the sprite to its center
  this.anchor.set(0.5);
  this.checkWorldBounds = true;
  this.outOfBoundsKill = true;
  //Record whether sprite is processed by the update() function
  this.exists = false;
  this.Hp = enemyHp;

}

//Enemy inherited from Phaser.sprite
Enemy.prototype = Object.create(Phaser.Sprite.prototype);
Enemy.prototype.constructor = Enemy;

Enemy.prototype.launch = function(x, y, angle, speed, xAccel) {
  //horizontal acceleration value
  xAccel = xAccel || 0;

  //Reset the Enemy, which moves the Enemy to the given x/y corrdinates and
  //sets 'exists' to true.
  this.reset(x, y);

  //Set Enemy's velocity that is calculated from the given angle and speed
  this.game.physics.arcade.velocityFromAngle(angle, speed, this.body.velocity);
  this.body.acceleration.x = xAccel;
}

var Enemy = function(game, key ,enemyHP) {
  //Call constructor of Phaser.Sprite to initialize this
  Phaser.Sprite.call(this, game, 0, 0, key);

  this.checkWorldBounds = true;
  this.outOfBoundsKill = true;
  //Record whether sprite is processed by the update() function
  this.exists = false;
  this.Hp = enemyHP;
}

//Enemy inherited from Phaser.sprite
Enemy.prototype = Object.create(Phaser.Sprite.prototype);
Enemy.prototype.constructor = Enemy;

Enemy.prototype.launch = function(x, y, angle, speed, xAccel) {
  //horizontal acceleration value
  xAccel = xAccel || 0;

  //Reset the Enemy, which moves the Enemy to the given x/y corrdinates and
  //sets 'exists' to true.
  this.reset(x, y);

  //Set Enemy's velocity that is calculated from the given angle and speed
  this.game.physics.arcade.velocityFromAngle(angle, speed, this.body.velocity);
  this.body.acceleration.x = xAccel;
}


Update enemy.js