js

JavaScript

https://developer.mozilla.org/en-US/docs/Web/JavaScript

• Recognized for
  • First-class functions
  • Weak typing
  • Prototypes
  • Automatic semicolon comp.

History

Developed by Brenden Eich of Netscape, initially named Mocha, then LiveScript, and finally JavaScript.

Variables

naming convention: camel case.

• var (the old way and should be avoided)
  • Global or function scope (due to hoisting)
  • Hoisted, declared, and not initialized, causing unexpected behaviors.
  • Redeclaration allowed (old value maintained unless assigned), causing confusion
  • Should be avoided since ES6
• let
  • Block scope
  • Redeclaration not allowed, reassignment allowed
• const
  • Block scope
  • Redeclaration and reassignment not allowed
• Without keyword
  • Will be in global scope (even if inside a block)
• Hoisting
  • = Raise something up with ropes and pulleys
  • Move the declaration up (but not the initialization)

console.log(x); // will output undefined
var x = 7;

• ReferenceError vs undefined
• When entering a function, JS will search and execute all the vars and declare them all.

Operators

• == returns true for “same thing”, === also requires “same type”. Always use === unless == is intended. (type coercion)

Functions

function func() { ... };
const func = function () { ... };
const func () => { ... };  // since ES6

All functions are hoisted.

Data Types

Primitives

• Boolean
• Number
• BigInt - const x = 111111n
• String - double quote, single quote, backtick (template string, since es6)
• Undefined type
• Symbol type

Objects

• Prototype-based object-oriented programming
• person.name and person["name"] both works
• Context: this.name
• const func = person.method.bind(person). Calling func = person.method will cause many undefined, since