Main Features

• object-oriented dynamic scripting language
• prototype based inheritance
• interpreted and JIT compiled
• single-threaded : Core JS has 1 call stack

JS Engine :

• program that executes code and converts it into machine readable format.
• Popular - V8 (Chromium , NodeJS), Spidermonkey (firefox)
• V8 engine is an open source high-performance, written in C++. It combines interpreter and compiler together

Parts of Engine

• Parser : scans script, checks errors and breaks it up into tokens. It creates a AST — a tree graph of source code
• Interpreter : used at 1st execution. Executes code line by line and converts them into bytecode
• Profiler: watches for frequently executed code (HOT code) and passes to compiler for optimisations and re-optimisations.
• JIT Compiler: compiles & optimise hot code into machine code. It is mixed with bytecode and given to computer.
• Engine Memory — made of 2 parts
  • heap memory to store objects in unordered way
  • stack
    • to store execution contexts, primitives and references to objects.
    • It is initialised with global context. Functions are ran by placing their contexts on stack.

Tips for optimised code

• when arguments are repeated, compiler may memoize