How to Change MouseConstraint Stiffness in Matter.js

Yes, you can change the stiffness of the spring used by a MouseConstraint in Matter.js. This article explains how the underlying constraint works and provides direct code examples showing how to configure the spring stiffness both during the creation of the mouse constraint and dynamically at runtime.

Understanding the Underlying Constraint

When you create a MouseConstraint using Matter.MouseConstraint.create(), Matter.js automatically generates a standard physics Constraint beneath the hood. This constraint binds the physics body under the cursor to the mouse pointer. Because it is a standard constraint, it inherits standard constraint properties, including stiffness and damping.

By default, the stiffness is set to a high value (typically 0.9 or 1), which makes grabbed bodies closely and rigidly track cursor movement. Reducing this value introduces elasticity, giving the interaction a noticeable spring or bungee effect.

Method 1: Setting Stiffness at Initialization

To set the stiffness when creating the MouseConstraint, pass a nested constraint object within the options parameter:

// Create the mouse constraint with custom stiffness
const mouseConstraint = Matter.MouseConstraint.create(engine, {
    constraint: {
        stiffness: 0.1, // Lower values (e.g., 0.05 to 0.2) create a softer, springier pull
        damping: 0.1,   // Optional: controls oscillation reduction
        render: {
            visible: true // Set to true to see the spring line
        }
    }
});

// Add the mouse constraint to the world
Matter.Composite.add(engine.world, mouseConstraint);

Method 2: Modifying Stiffness Dynamically at Runtime

If the MouseConstraint already exists, you can alter the stiffness on the fly by directly modifying the stiffness property on the nested constraint object:

// Dynamically adjust stiffness after creation
mouseConstraint.constraint.stiffness = 0.05;

Tips for Spring Tuning