// going to use this to control horizontal velocity of player
var playerController = new Body(BodyType.STATIC);
playerController.space = space;

// LineJoint is used to match the playerController horizontal kinematicVel to the players horizontal velocity.
// we set stiff = false and maxError = 0 to disable the positional part of the constraint
// and so the anchors really don't matter (hence just set to (0,0) here)
var playerJoint = new LineJoint(playerController, player, Vec2.weak(), Vec2.weak(), Vec2.weak(0,1), Math.NEGATIVE_INFINITY, Math.POSITIVE_INFINITY);
playerJoint.stiff = false;
playerJoint.maxError = 0;
playerJoint.space = space;

...

before calling space.step()
playerController.kinematicVel.x = whateverXVelocityYouWouldLikePlayerToHave;