Multiply a Vector by a Scalar in Ammo.js

This article explains how to correctly multiply a 3D vector by a numeric scalar in the ammo.js physics library. You will learn the correct syntax using both the built-in Emscripten operator binding and an alternative in-place multiplication method designed to prevent memory leaks in JavaScript.

Using the op_mul Method

In ammo.js (which is a direct Port of the Bullet physics engine via Emscripten), standard C++ mathematical operators are exposed as methods. The multiplication operator (*) for a btVector3 and a float scalar is bound to the op_mul method.

// Create a vector and define a scalar
const vector = new Ammo.btVector3(1.0, 2.0, 3.0);
const scalar = 2.5;

// Multiply the vector by the scalar
const scaledVector = vector.op_mul(scalar);

console.log(scaledVector.x(), scaledVector.y(), scaledVector.z()); // Outputs: 2.5, 5.0, 7.5

Memory Management Caution

Because op_mul returns a brand new btVector3 instance allocated on the WebAssembly heap, you must manually destroy the resulting vector when you are done using it to avoid memory leaks.

// Clean up the allocated vectors
Ammo.destroy(vector);
Ammo.destroy(scaledVector);

The In-Place Multiplication Alternative (Recommended)

To avoid garbage collection overhead and potential memory leaks caused by creating new objects in high-frequency loops (such as your game’s update loop), you can modify the original vector’s components in place using setValue().

const vector = new Ammo.btVector3(1.0, 2.0, 3.0);
const scalar = 2.5;

// Multiply in-place without allocating new memory
vector.setValue(
    vector.x() * scalar,
    vector.y() * scalar,
    vector.z() * scalar
);

console.log(vector.x(), vector.y(), vector.z()); // Outputs: 2.5, 5.0, 7.5

// Only destroy the original vector when it is no longer needed
Ammo.destroy(vector);

Using the in-place method is the best practice for performance-critical WebGL applications, as it completely bypasses the creation of temporary WebAssembly objects.