We’ve recently added support for the FinalizationRegistry API in Cloudflare Workers. This API allows developers to request a callback when a JavaScript object is garbage-collected, a feature that can be particularly relevant for managing external resources, such as memory allocated by WebAssembly (Wasm). However, despite its availability, our general advice is: avoid using it directly in most scenarios.

Our decision to add FinalizationRegistry — while still cautioning against using it — opens up a bigger conversation: how memory management works when JavaScript and WebAssembly share the same runtime. This is becoming more common in high-performance web apps, and getting it wrong can lead to memory leaks, out-of-memory errors, and performance issues, especially in resource-constrained environments like Cloudflare Workers.

In this post, we’ll look at how JavaScript and Wasm handle memory differently, why that difference matters, and what FinalizationRegistry is actually useful for. We’ll also explain its limitations, particularly around timing and predictability, walk through why we decided to support it, and how we’ve made it safer to use. Finally, we’ll talk about how newer JavaScript language features offer a more reliable and structured approach to solving these problems.

...read more at blog.cloudflare.com