Toptal

As a developer, I naturally want my software to be reliable and responsive. In the early days of my career, feedback on my applications was mixed. Some apps scored high praise, but reviews were inconsistent on other apps because they would intermittently stop responding midsession—and we all know how little patience end users have for poor program responsiveness.

The underlying issue was that the apps were coded using purely synchronous JavaScript. Since JavaScript offers (seemingly) asynchronous functions, it’s easy to miss the fact that JavaScript’s runtime itself is synchronous by default, and this is a potential pitfall for developers. My curiosity drove me to investigate this programmatic puzzle.

The Problem: JavaScript Synchronous Blocking

I started my exploration by observing the way that regular, synchronous calls work, focusing my efforts on call stacks—last in, first out (LIFO) programming structures.

All call stacks function alike, regardless of the language: We push (add) function

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