Why Manual Memory Management in Ammo.js WebAssembly

Ammo.js, a WebAssembly (Wasm) port of the Bullet physics engine, requires developers to manually delete allocated math objects using Ammo.destroy(). While JavaScript is a garbage-collected language, the WebAssembly virtual machine operates on a separate, linear memory heap that the JavaScript garbage collector cannot access. This article explains the technical reasons behind this architectural separation, how memory leaks occur when handling math objects, and how to properly manage memory to ensure high-performance physics simulations.

The WebAssembly and JavaScript Memory Divide

The primary reason developers must manually free Ammo.js objects lies in how WebAssembly manages memory. WebAssembly runs inside a sandboxed environment with a single, contiguous array of raw bytes called linear memory.

When you instantiate an Ammo.js object, such as a vector or a transform:

const vector = new Ammo.btVector3(0, 1, 0);

Two distinct allocations occur: 1. In WebAssembly Memory: The actual C++ data structure (the three 32-bit floats for X, Y, and Z) is allocated within WebAssembly’s linear memory heap. 2. In JavaScript Memory: A lightweight JavaScript wrapper object is created. This wrapper merely holds a pointer (a memory address integer) pointing to the location of the C++ object inside the WebAssembly heap.

Why JavaScript Garbage Collection is Insufficient

The JavaScript garbage collector (GC) only monitors the JavaScript heap. It tracks references to the lightweight JS wrapper object.

If the wrapper object goes out of scope, the JavaScript engine will eventually garbage collect it. However, the JS GC has no awareness of, or access to, the WebAssembly linear memory heap. It does not know that the JS wrapper was pointing to allocated C++ memory.

Consequently, when the JS wrapper is destroyed, the underlying C++ object inside the WebAssembly heap remains allocated. Because the pointer to this memory is now lost, that chunk of WebAssembly memory becomes permanently unreachable—creating a memory leak.

The Impact on Math Objects in Physics Loops

While memory leaks are problematic in any application, they are particularly catastrophic with math objects like btVector3, btQuaternion, and btTransform in physics engines.

Physics calculations run inside a rendering loop, typically executing 60 times per second. If a developer allocates temporary math objects within this loop to calculate forces, velocities, or raycasts without freeing them, the application will leak hundreds of objects every second.

Because the WebAssembly heap is statically sized and can only grow up to a hard limit, the application will quickly run out of memory. This leads to severe performance degradation, stuttering, and eventual application crashes.

How to Properly Manage Ammo.js Memory

To prevent memory leaks, developers must adopt one of two strategies: manual destruction or object reuse.

1. Manual Destruction with Ammo.destroy()

When you are finished using a temporary Ammo.js object, you must explicitly free its WebAssembly memory using Ammo.destroy():

function applyForceToPlayer(playerBody) {
    const force = new Ammo.btVector3(0, 10, 0); // Allocated on Wasm heap
    playerBody.applyCentralForce(force);
    
    Ammo.destroy(force); // Explicitly frees the Wasm memory
}

2. Object Reuses (Caching)

Because allocating and destroying objects continuously in a 60 FPS loop introduces CPU overhead, the most efficient pattern is to instantiate math objects once and reuse them.

// Allocate once globally or in a scoped class
const tempVector = new Ammo.btVector3();

function updatePhysics(playerBody) {
    // Modify the existing object instead of creating a new one
    tempVector.setValue(0, 10, 0); 
    playerBody.applyCentralForce(tempVector);
}

// Cleanup only when the application or scene closes
function cleanup() {
    Ammo.destroy(tempVector);
}

By understanding the division between the JavaScript and WebAssembly heaps, developers can write robust, leak-free physics integrations that maintain high frame rates over extended sessions.