Object pooling.js

// declare bullet pools
var activeBullets = [];
var bulletPool = [];

// construct some bullets ready for use
for (var i=0; i < 20; i++)
    bulletPool.push( new Bullet() );

// a constructor/factory function
function getNewBullet()
{
    var b = null;

    // check to see if there is a spare one
    if (bulletPool.length > 0)
        b = bulletPool.pop();
    else 
        // none left, construct a new one
        b = new Bullet();	

    // move the new bullet to the active array
    activeBullets.push(b);
    return b;
}

function freeBullet(b)
{
    // find the active bullet and remove it
    // NOTE: Not using indexOf since it wont work in IE8 and below
    for (var i=0, l=activeBullets.length; i < l; i++)
        if (activeBullets[i] == b)
            array.slice(i, 1);
	
    // return the bullet back into the pool
    bulletPool.push(b);
}

From http://buildnewgames.com/garbage-collector-friendly-code/

When you want to fire a bullet, instead of doing:

var b = new Bullet();

You use the factory function:

var b = getNewBullet();

This will avoid construction by just returning one of the pre-constructed objects from the array, or if there are none available, fall back to constructing another bullet - essentially automatically expanding the pool.

Once you’re done with the bullet, you need to return it to the pool:

if (bullet.collidingWith(enemyShip))  // boom!
  freeBullet(bullet); // return it to the pool