A JavaScript engine is a program or a component of a web browser that executes JavaScript code. It is responsible for interpreting and executing the code written in JavaScript. JavaScript engines typically have several components, including a parser, a compiler, and an interpreter.
The parser is responsible for breaking down the JavaScript code into tokens, which are individual units of meaning. The compiler converts the tokens into executable bytecode, which is a lower-level representation of the code. Finally, the interpreter executes the bytecode.
JavaScript engines also include various optimization techniques to improve performance, such as inlining functions, eliminating dead code, and reordering code for better cache performance. Additionally, some engines use a Just-In-Time (JIT) compiler to dynamically compile the bytecode into machine code on-the-fly, which can result in significant performance gains.
JavaScript engines are used in web browsers to execute JavaScript code on the client-side. They are also used in server-side environments, such as Node.js, to execute JavaScript code on the server. Different web browsers and environments use different JavaScript engines, each with their own strengths and weaknesses. Some popular JavaScript engines include V8 (used in Google Chrome and Node.js), SpiderMonkey (used in Mozilla Firefox), and JavaScriptCore (used in Apple Safari).