three;js: Error trying to access class after adding information to it

Playing around with classes / prototypes. Basically I'm trying to get a 3d cube to face the direction the mouse is in. I think it's almost there, but right now it's not rendering the cube, which it was doing fine before I added this code:

cube.look(xTarget, yTarget);

which is giving this error:

 Uncaught TypeError: Cannot read property 'look' of undefined`

What am I doing wrong here? Why is it unable to access the var cube? (which is what I think the problem is).

Here's a plunkr

Here's the relevant js:

Cube.prototype.updateBody = function(speed){
    this.box.rotation.y += (this.tBoxRotY - this.box.rotation.y) / speed;
    this.box.rotation.x += (this.tBoxRotX - this.box.rotation.x) / speed;
    this.box.position.x += (this.tBoxPosX-this.box.position.x) / speed; 
    this.box.position.y += (this.tBoxPosY-this.box.position.y) / speed; 
    this.box.position.z += (this.tBoxPosZ-this.box.position.z) / speed; 
}

Cube.prototype.look = function(xTarget, yTarget){
    this.tBoxRotY = rule3(xTarget, -200, 200, -Math.PI/4, Math.PI/4);
    this.tBoxRotX = rule3(yTarget, -200,200, -Math.PI/4, Math.PI/4);
    this.tBoxPosX = rule3(xTarget, -200, 200, 70,-70);
    this.tBoxPosY = rule3(yTarget, -140, 260, 20, 100);
    this.tBoxPosZ = 0;
}

function loop() {
      render();
    var xTarget = (mousePos.x-windowHalfX);
    var yTarget= (mousePos.y-windowHalfY);
    console.log('Mouse X position: ' + xTarget +', Y Target = '+yTarget );
    cube.look(xTarget, yTarget);
    requestAnimationFrame(loop);
}

via Chebli Mohamed