In February of 2010, I was on the phone with Jason Yan, CTO and cofounder of a web startup called Disqus. At the time, Disqus was a tiny company behind a fast-growing commenting application, distributed as a third-party script and popular with bloggers and a handful of large media companies. Jason was interviewing me for a JavaScript engineering role—their first hire dedicated to working on a fast-growing client codebase.

After a handful of standard JavaScript interview questions involving classes, prototypes, and scopes, Jason took a different tack. He asked me the following (roughly paraphrased) question: “Let’s say I’ve taken a native function prototype—like Array.prototype.indexOf and assigned it a new value. How would you get the original value back?”

I was dumbstruck. This was a problem I had never encountered before, and I didn’t know the answer. Jason explained to me that the Disqus application executes in environments they don’t control. And in those environments, native properties are sometimes overwritten or mangled, properties that they depend on.

I wasn’t about to give up on the question. So in the middle of the interview, I opened up my browser’s JavaScript console, and started fiddling around with function prototypes. In a few short minutes, I made the startling discovery that you could use JavaScript’s delete operator on a modified native property, and the browser would restore the original value.

Jason, as it turns out, was unaware of this solution. He tried out the technique himself and, sure enough, it worked. We were equally excited at this new discovery. We began talking, discussing Disqus’ current solution to this problem, and the interview changed from a serious interrogation into an excited conversation about iframes, browser hacks, and other scripting gotchas.

I didn’t know it at the time, but this was my first taste of Third-party JavaScript; of solving problems that only affect client applications running in other people’s web environments; of discovering techniques and practices that some web developers may never be aware of. And I was hooked.

Several more interviews and two months later, I joined the Disqus team, which only had seven employees at the time, in San Francisco. That was where I met Anton Kovalyov, my new coworker, fellow JavaScripter, and future coauthor. For the next two-plus years, Anton and I were responsible for maintaining and developing Disqus’ client-side code. Disqus continued to grow—rapidly. By 2012, it was installed on hundreds of thousands of web pages, and received over 5 billion page views per month. Its customers included CNN, MLB, IGN, Time.com, Rolling Stone, and dozens of other major web and media properties.

During that period of time, Anton and I learned dozens of helpful tricks, tips, and hacks for third-party scripters, most of which we learned the hard way, and some of which we kept under wraps intentionally, because they gave us a technological edge.

In this book, we’ve pooled our collective knowledge about third-party JavaScript. Not only do we think our book will help third-party scripters everywhere, we also think that the practices we discuss may help make the web a better place for everybody. We hope that by the end of reading it, you’ll agree.

Ben Vinegar