spring.js

/*
In Javascript, any function acts as an object constructor when you call it with the `new` keyword.
So, this:

    var spring1 = new Spring(particle1, particle2, length, stiffness);

creates a new object which inherits all of Spring.prototype's methods and properties, and then the function Spring(..) is called on it.
Inside the function Spring(..), `this` refers to the newly-created object.
*/

function Spring(particle1, particle2, length, stiffness)
{
	// Previously, you were accessing the parameters of the constructor by defining the object's methods in a closure.
	// Store them as properties of the objects instead.
	this.particle1 = particle1;
	this.particle2 = particle2;
	this.length = length;
	this.stiffness = stiffness;

        // you don't need to do this - use the `new` keyword wherever you call Spring() to create an instance of Spring.
	// It automatically inherits all of Object's methods.
	/*
	  spring = new Object();
	  return spring;
	*/
}
Spring.prototype = {

	force: function ()
	{
		var particle1 = this.particle1,
		    particle2 = this.particle2,
		    length = this.length;
		    stiffness = this.stiffness;

		x = Math.sqrt(Math.pow(particle2.position.x - particle1.position.x, 2) + Math.pow(particle2.position.y - particle1.position.y, 2)) - length;
		
		return stiffness * x;
	}
	
	act: function ()
	{
		// this.force() might change after applying the force to particle 1, depending on how it's done. 
		// Anyway, don't calculate it twice if you expect it to be the same value.
		var force = this.force();

		particle1.applyforce(particle1.position.to(particle2.position).unit().scale(force));
		particle2.applyforce(particle2.position.to(particle1.position).unit().scale(force));
	}
}