这是常规的构造函数吗？

Question

So I've been looking at a script for a game which I've downloaded. The thing I'm unsure about is Bullet(I) is never instantiated within the script (ie var x = new Bullet). The tutorial refers to this as a constructor function though. What is going on?

It looks as though the Bullet constructor is taking a parameter and adding properties etc to it. But nowhere in the script is Bullet ever instantiated - so it cannot be?

var playerBullets = [];

    function Bullet(I) {
      I.active = true;
      I.xVelocity = 0;
      I.yVelocity = -I.speed;
      I.width = 3;
      I.height = 3;
      I.color = "#000";

      I.inBounds = function() {
        return I.x >= 0 && I.x <= CANVAS_WIDTH &&
          I.y >= 0 && I.y <= CANVAS_HEIGHT;
      };

      I.draw = function() {
        canvas.fillStyle = this.color;
        canvas.fillRect(this.x, this.y, this.width, this.height);
      };

      I.update = function() {
        I.x += I.xVelocity;
        I.y += I.yVelocity;

        I.active = I.active && I.inBounds();
      };

      I.explode = function() {
        this.active = false;
        // Extra Credit: Add an explosion graphic
      };

      return I;
    }

This code is used later in the script and as far as I can see, this has to be the relevant part of the script that is using the Bullet(I) function?

playerBullets.forEach(function(bullet) {
    bullet.update();
});

Answer 1

I wouldn't personally call that a constructor in the usual sense where, as you correctly state, you would use it like var myBullet = new Bullet() .

It does however do a similar job of "constructing" and object by using javascripts loose typing to be able to add properties and methods to an object at run-time. This method would be used like:

var myBullet = {}; // blank object
Bullet(myBullet);
// myBullet now has methods .draw, .active etc

As I have discovered the code you have downloaded came from this tutorial and the method Bullet is indeed called on line 167 of index.html. It configures a bullet when the player shoots:

player.shoot = function() {
      Sound.play("shoot");

      var bulletPosition = this.midpoint();

      playerBullets.push(Bullet({ //<-- here
        speed: 5,
        x: bulletPosition.x,
        y: bulletPosition.y
      }));
    };

Answer 2

It appears that Bullet is taking a (presumably empty) object as a parameter and then turning it into a an object of Bullet "class" (yes, I know javascript doesn't have classes, but you know what I mean). I suspect the reason for this is to ensure objects are reused rather than being created and destroyed. This means less garbage collection takes place and so the game runs more smoothly

Answer 3

It seems, that it takes an -probably- object, namely "I", as an input, and attaches stuff to it and returns that. No need to instantiate Bullet itself. You only need proper I(nput)objects. At best it should be called something like "builder function".