Of races and mutexes: synchronizing async operations in JavaScript

While JavaScript is a strictly single-threaded language, the asynchronous nature of its execution model can easily lead to situations in which two or more async operations run at the same time and compete with the program flow depending on which operation succeeds first. The result is a specimen of the dreaded species of race conditions.

Issues of this type can be arbitrarily hard to reproduce, debug and fix, and I guess that every seasoned JavaScript developer can relate one or two horror stories where they literally spent days before finally fixing such an issue for good.

Race conditions are a well-known problem from any language that supports for concurrency, and often a mutex is used as a tool to synchronize such operations. This article shows how to apply the concept of mutexes to JavaScript in order to avoid race conditions.