JavaScript Engines

Source: What are JavaScript Engines? - DEV Community, JavaScript engine - Wikipedia

What are JavaScript Engines

• JavaScript engine is like a virtual machine (VM)
  • Virtual machine is software the emulates a computer system
    • System virtual machine:
      • Examples: Parallels on Mac to run Windows, VirtualBox to run operating systems on Windows, Mac, Linux
    • Process virtual machine: runs one program/process
      • Examples: Wine that runs Windows applications on a Linux computer, but does not provide all functionality of a Windows operating system
• JavaScript (JS) engine is a kind of process virtual machine to interpret and execute JavaScript code
  • In summary, the JavaScript engine takes JavaScript code written by a developer and converts it to optimized code that can be run fast and interpreted by a browser or inside an application
• Each JavaScript engine implements a version of ECMAScript. JavaScript is a dialect of ECMAScript. ECMAScript engine is another name for the implementations.
  • Each engine is design to work with a different web browser (examples like regular or headless Chrome, Firefox) or runtime like Node.js
  • Example engines:
    • WebKit’s JavaScriptCore which has 6 building blocks that analyze, interpret, optimize, and garbage collect JavaScript code
    • Google’s V8 engine

How do Engines Work

• Different engines do code processing differently
• Example: JavaScriptCore
  • Lexical analysis: source is broken into tokens or strings
  • Tokens are analyzed by parser for syntax and built into a syntax tree
  • 4 Just in Time (JIT) processes start: “JavaScript engine takes your source code, breaks it up into strings (a.k.a. lexes it), takes those strings and converts them into bytecode that a compiler can understand, and then executes it.”
• The machine code made in the compilation process allows to engine to expose data types, operators, objects, and functions to the browser or runtime (example Node.js, Deno, Electron)

Runtimes and Engines

• Popular runtimes work on top of JavaScript engines and extend the engine’s power
  • Examples: Node.js, Deno, Bun
    • Node.js and Deno embed V8
    • Bun embeds JavaScriptCore
• Runtimes are built to solve challenges with JavaScript’s single thread architecture and look to improve performance of the engines that it uses
  • For example, Node prioritizes asynchronous, threadless execution of routines

Browser, Runtime, Other	JavaScript Engine
Mozilla	Spidermonkey
GNOME Shell extensions	Spidermonkey
Chrome	V8
IE	Chakra
Safari	JavaScriptCore
Node.js	V8
Deno	V8
Bun	JavaScriptCore
Edge	Blink and V8

How to apply Knowledge of Engines

• Goal of JavaScript engines creating the most fast execution in the shortest time
• Developers need to be aware of:
  • Differences in browsers and code performance
  • Limits and possibilities given engines and devices and effects on user experience