How to Stop planck.js Body Rotation?

In planck.js (the JavaScript port of the Box2D physics engine), you often need a rigid body that can move freely in the 2D plane but must remain perfectly upright or oriented in a fixed direction. This article provides a quick overview of how to lock a body’s rotation, detailing the most efficient property-based method, alternative joint-based workarounds, and how to dynamically toggle this behavior during gameplay.

The Best Method: Using fixedRotation

The cleanest and most computationally efficient way to prevent a planck.js body from rotating is by setting its fixed rotation flag to true. This instructs the physics solver to completely ignore any rotational forces (torques) acting on the body while leaving its linear velocity and forces unaffected.

Setting Fixed Rotation During Initialization

When creating your physics body, you can pass fixedRotation: true directly inside the body definition object.

const body = world.createBody({
  type: 'dynamic',
  position: planck.Vec2(0, 5),
  fixedRotation: true // Prevents any rotation from the start
});

Setting Fixed Rotation Post-Creation

If you need to change this behavior after the body has already been instantiated, planck.js provides a built-in setter method.

// Lock the rotation later in your code
body.setFixedRotation(true);

// Unlock the rotation to allow the body to spin again
body.setFixedRotation(false);

Alternative Method: Using a Prismatic Joint

While fixedRotation is almost always the preferred solution, specialized game mechanics might require you to restrict rotation relative to another moving body rather than the world axis. In these rare scenarios, you can use a Prismatic Joint.

A Prismatic Joint restricts the relative motion of two bodies along a specified axis and completely prevents relative rotation between them.

const jointDef = {
  anchorA: bodyA.getWorldCenter(),
  localAxisA: planck.Vec2(1, 0), // Allows movement along this axis
  enableLimit: false
};

world.createJoint(planck.PrismaticJoint(jointDef, bodyA, bodyB));

Common Pitfalls to Avoid