I’ve been using TypeScript in a side project recently and was learning about the JavaScript event loop. As a Pythonista, it made me think of the Python GIL. Both the event loop and the GIL are concurrency mechanisms, but they work very differently.

The GIL allows only one thread to execute Python bytecode at a time. This prevents true multi-threading: even though multiple threads can exist at the same time, they have to take turns executing Python bytecode. The GIL exists because CPython’s memory management isn’t thread safe.

The event loop is single-threaded by design: there is only one thread running JavaScript code. It uses an event-driven, non-blocking I/O model. Concurrency comes from the ability to pause and resume functions and handle I/O asynchronously.

Both the event loop and the GIL prevent multiple pieces of code from running simultaneously, but the GIL is a limitation whereas the event loop is a design choice.

Definitions

• I/O refers to operations that interact with systems outside of the JavaScript engine. This is anything that involves waiting for external resources, such as API calls, HTTP requests, reading/writing files, reading/writing databases, and more.
• Concurrency means that multiple operations can be in progress at the same time, even though only one piece of JavaScript code (or Python bytecode) executes at any given time.