Modern web applications load and execute a lot more JavaScript code than they did just a few years ago. While JIT (just-in-time) compilers have been very successful in making JavaScript performant, we needed a better solution to deal with these new workloads.

To address this, we’ve added a new, generated JavaScript bytecode interpreter to the JavaScript engine in Firefox 70. The interpreter is available now in the Firefox Nightly channel, and will go to general release in October. Instead of writing or generating a new interpreter from scratch, we found a way to do this by sharing most code with our existing Baseline JIT.

The new Baseline Interpreter has resulted in performance improvements, memory usage reductions and code simplifications. Here’s how we got there: