Just-in-time (JIT) compilers

Between 2008 and 2009, Mozilla, the company behind Firefox, slowly introduced TraceMonkey, the first Just-in-time (JIT) compiler for JavaScript, in different versions of Firefox 3.x, starting with 3.1. Similarly, the V8 JavaScript Engine, which powers Chrome and Safari, and Chakra, which powers Internet Explorer and Edge, also included a JIT compiler.

Traditionally, the JavaScript engine uses an interpreter, which translates the JavaScript source code into machine code that your computer can run. The JIT compiler improved the performance of the engine by identifying blocks of code that are frequently run, compiling them, and adding them to a cache. When the same block of code needs to be run again at a later time, the JavaScript engine can simply run the cached, pre-compiled machine code, skipping the interpreter altogether. Needless to say, this is much faster and the JavaScript engine can execute more operations per unit time, greatly increasing performance.