Why Ammo.js Requires Manual Memory Management

This article explains why developers must manually manage memory when using ammo.js, a popular 3D physics engine for the web. While JavaScript typically handles memory automatically through garbage collection, ammo.js operates under a different architecture that bypasses these standard mechanisms, making explicit memory deallocation necessary to prevent severe performance degradation and browser crashes.

The Emscripten and WebAssembly Wrapper

At its core, ammo.js is not written in native JavaScript. It is a direct port of the Bullet Physics SDK, which is written in C++. To run this C++ code in a web browser, developers use a toolchain called Emscripten to compile the source code into WebAssembly (Wasm) or asm.js.

When the compiled C++ code runs in the browser, it operates within a sandboxed, pre-allocated memory space called the WebAssembly heap. This heap is separate from the standard JavaScript engine’s memory heap.

The Limits of JavaScript Garbage Collection

JavaScript features automatic garbage collection, meaning the browser automatically detects when an object is no longer referenced and frees its memory. However, the JavaScript garbage collector can only see and manage native JavaScript objects.

When you instantiate an ammo.js object—such as a physics body, collision shape, or vector—using code like new Ammo.btVector3(), two things happen: 1. A small wrapper object is created in the JavaScript memory heap. 2. The actual physics data and memory structure are allocated within the WebAssembly/C++ heap.

If you lose the reference to the JavaScript wrapper object (for example, by letting it go out of scope), the browser’s garbage collector will eventually destroy the JS wrapper. However, the garbage collector has no visibility into the WebAssembly heap. Consequently, the actual C++ object remains allocated in the Wasm memory space forever, leading to a memory leak.

The Consequences of Memory Leaks in Physics Engines

Physics simulations are highly repetitive, often updating 60 times per second. If you allocate temporary vectors, transform matrices, or collision objects inside an active render loop without freeing them, the WebAssembly heap will rapidly fill up.

Once the WebAssembly memory allocation reaches its limit, the browser will throw an “out of memory” error, causing the entire physics simulation—and potentially the browser tab—to crash.

How to Manage Memory Manually in Ammo.js

To prevent these leaks, you must explicitly tell ammo.js to free the C++ side of the allocated memory when an object is no longer needed. This is achieved using the Ammo.destroy() function.

// Allocation
const forceVector = new Ammo.btVector3(0, -9.8, 0);

// Use the object in your physics simulation
rigidBody.applyCentralForce(forceVector);

// Manual deallocation once finished
Ammo.destroy(forceVector);

By calling Ammo.destroy(), you delete the object from the WebAssembly heap, keeping the memory footprint stable. For complex applications, developers often implement object pools for frequently created items (like vectors and matrices) to reuse them instead of constantly allocating and destroying memory.