How to Get Joints Connected to a Planck.js Body

This article provides a quick overview of how to access and iterate through the list of joints attached to a specific rigid body in planck.js, the 2D JavaScript physics engine based on Box2D. You will learn about the built-in methods available on the Body object, see a practical code example, and understand how to safely manipulate or inspect connected joints during your simulation.

Accessing Connected Joints via the Joint Edge List

In planck.js, joints are not stored as a simple, flat array directly inside a Body object. Instead, the engine uses a linked list structure called a Joint Edge (JointEdge) list to connect bodies and joints efficiently.

To retrieve the joints attached to a specific body, you must traverse this list starting from the body’s initial joint edge.

The getJointList() Method

The Body prototype provides a method called getJointList(). This method returns the first JointEdge object connected to that body, or null if the body has no attached joints.

Each JointEdge object contains three essential properties:

• joint: The actual Joint object (e.g., RevoluteJoint, DistanceJoint).
• other: The other Body connected to this joint.
• next: A pointer to the next JointEdge in the linked list, or null if it is the end of the list.

Code Example: Iterating Through a Body’s Joints

To inspect or manipulate all joints connected to a body, you can use a while loop to traverse the linked list.

// Assume 'myBody' is a planck.Body instance already added to your world
let edge = myBody.getJointList();

while (edge !== null) {
    // Access the current joint
    let joint = edge.joint;
    
    // Access the other body connected via this joint
    let connectedBody = edge.other;
    
    console.log(`Joint Type: ${joint.getType()}, Connected to Body:`, connectedBody);
    
    // Move to the next joint edge in the linked list
    edge = edge.next;
}

Important Considerations

• Do Not Modify the List Mid-Iteration: If you plan to destroy joints while looping through the list, destroying a joint immediately modifies the linked list pointers (edge.next). To avoid breaking your loop or causing null-pointer exceptions, always store a reference to the next edge (let nextEdge = edge.next) before destroying the current joint.
• Bi-directional Connection: Because joints connect two bodies, the joint will appear in the joint list of both connected bodies. The edge.other property is highly convenient because it automatically points to the opposite body relative to the one you called getJointList() on.