Crap sphere collision with ray library

collide.js

console.log("collide.js loaded");

function Ray(dx, dy, dz, x, y, z){
    if (x === undefined){x = 0};
    if (y === undefined){y = 0};
    if (z === undefined){z = 0};
    this.dx = dx;
    this.dy = dy;
    this.dz = dz;
    this.x = x;
    this.y = y;
    this.z = z;
}

function Sphere(x, y, z, r){
    this.x = x;
    this.y = y;
    this.z = z;
    this.r = r;
}

Sphere.prototype = {
    getIntersections : function(ray){
        var x1 = ray.x;
        var y1 = ray.y;
        var z1 = ray.z;
        var x2 = ray.x + ray.dx;
        var y2 = ray.y + ray.dy;
        var z2 = ray.z + ray.dz;
        var x3 = this.x;
        var y3 = this.y;
        var z3 = this.z;
        var r  = this.r;

        var pow = Math.pow

        var a = pow(x2 - x1, 2) + pow(y2 - y1, 2) + pow(z2 - z1, 2);
        var b = 2*((x2 - x1)*(x1 - x3) + (y2 - y1)*(y1 - y3) + (z2 - z1)*(z1 - z3) );
        var c = x3 * x3 + y3 * y3 + z3 * z3 + x1 * x1 + y1 * y1 + z1 * z1 - 2*(x3*x1 + y3*y1 + z3*z1) - r * r;
        var radicand = b * b - 4*a*c

        var xyz_from_u = function(u) {
            return [x1 + u * (x2 - x1), y1 + u * (y2 - y1), z1 + u * (z2 - z1)];
        }

        if (radicand < 0){
            return [];
        } else if (radicand == 0){
            return [xyz_from_u(-b / (2*a))];
        } else if (radicand > 0){
            return [xyz_from_u((-b + Math.sqrt(radicand)) / (2*a)), xyz_from_u((-b - Math.sqrt(radicand)) / (2*a))];
        } else {
            throw "logic error!";
        }
    }
};


function assertEqual(thing1, thing2){
    if (thing1 !== thing2){
        throw {message: ""+thing1+" is not equal to "+thing2, name : "Failed Test"};
    } else {
        console.log("test passed: "+thing1+" is equal to "+thing2);
    }
}

function test_sphere_intersection(){
    var sphere = new Sphere(4,0,0,1);
    var ray = new Ray(1,0,0);
    var intersections = sphere.getIntersections(ray);
    intersections.sort();
    assertEqual(intersections[0][0], 3);
    assertEqual(intersections[0][1], 0);
    assertEqual(intersections[0][2], 0);
    assertEqual(intersections[1][0], 5);
    assertEqual(intersections[1][1], 0);
    assertEqual(intersections[1][2], 0);
}

test_sphere_intersection();

I also have some minor comments.
line 35: I find that 'static function imports' like this (that's effectively what you're doing) are most readable at the top of the program.

line 53: I assume this is just in case you get a DNE number or some weird javascript type-ing? Might be worth throwing a slightly more descriptive error.

I think some of the math you're doing in lines 37-39 would be more readable if you created some more separate functions. Although you'll be making your code more verbose to do so, so that's up to you. Some that I would consider creating, a 'square' function (maybe call it sq()), something to do this for you: (y2 - y1)*(y1 - y3) given 3 variables.

Final minor point: I think you exceeded 80 columns (or is it 100 in github?) in line 39 which forces readers to use their scroll buttons (the horror!).