🐡 Engine

• JS is object-oriented dynamic scripting language. It is JIT compiled and single-threaded.

• JS Engine is a 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, create tokens and AST.
• Interpreter : used at 1st execution. Executes code line by line → 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 Heap and Stack. Heap store objects in unordered way. While Stack stores 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

• compiler may memoize — repeated args, returns from pure functions
• avoid arguments eval for in delete
• hidden classes : get attached to every object to track its shape/layout. Helps compiler optimize the property access time.