Most JS runtimes are multi-threaded behind the scenes. If you start a node process: node -e "setTimeout(()=>{}, 10_000)" & Then wait a second and run: ps

Most JS runtimes are multi-threaded behind the scenes. If you start a node process: node -e "setTimeout(()=>{}, 10_000)" & Then wait a second and run: ps -o thcount $! Or on macOS: ps -M $! You'll see that there are multiple threads running, but like others have said, it's completely opaque to you as a programmer. It's basically just an implementation detail.