Conceptually, what happened was foo.a = a; foo.b = await b(); // while waiting for b's completion, something happens that changes the value of `a`, making

Conceptually, what happened was foo.a = a; foo.b = await b(); // while waiting for b's completion, something happens that changes the value of `a`, making `foo` invalid (or more complex variants of this) In a multi-threaded model, this would be classified as a race condition on `a`. That's why Rust has RefCell for r/w data sharing in single-threaded code. I don't remember the exact details, but I was hit by this many times when refactoring e.g. db access or file access in the (JS) code of Firefox. This can happen without async/await, without Promise and even without an event loop, you just need callbacks. But of course, having an event loop, Promise and async/await give you way more opportunities to hit such an issue.