Javascript

• Specified in ECMA 262
• Level of abstraction: $$JS/Python > C/C++ > Assembly > Machine Code$$

Styles

CJS (Common JavaScript)

• Load modules synchronously

// exporting
modules.exports = {
  myFunction
}

// importing
const { myFunction } = require('./file.js')

ESM (ECMAScript Module)

• Available since ES2015 (ES6?)
• Supports loading modules asynchronously
• Run implicitly in strict mode
• Requires {"type":"module"} in package.json
• The complete filename must be specified. E.g., 'file.js' (instead of 'file')

// exporting
export {
  myFunction
}

// importing
import { myFunction } from './file.js'

Code Translation

• Interpreter: read the source code and execute it
• Compiler: statically analyze all the code in advance and compile it to a binary that runs by itself on the machine

Implementations

• Mozilla Firefox: Spider Monkey
• Google Chrome: V8

Characteristic

• Interpreted
• Dynamic weakly typed: data type become known at runtime
• Multi-paradigm: declarative functional or imperative object-oriented
• Prototype inheritance: everything in js in an object with prototype methods inherited
• Single-threaded: one computation at a time
• Garbage-collected: clear free memory in heap when it's no longer necessary
• Non-blocking
• Event Loop (JIT)
• For each round of the event loop
  1. Run sync code
  2. Run microtask callbacks (from callback queue): e.g., Promises
  3. Run macrotask callbacks (from callback queue): async task callbacks

// 1st
console.log("sync1");

// 4th
setTimeout((_) => console.log("timeout"), 0);

// 3rd
Promise.resolve().then((_) => console.log("promise"));

// 2nd
console.log("sync2");