1. Data types in JavaScript

JavaScript is dynamically typed: a variable's type is decided at runtime and can change.

Primitive types (immutable, stored by value)

Type	Example
number	42, 3.14, NaN, Infinity
string	"hello", 'hi', `template ${x}`
boolean	true, false
undefined	a declared variable with no value
null	intentional "no value"
symbol	Symbol('id'), a unique identifier
bigint	123n, for very large integers

Non-primitive (reference) type: object, which includes plain objects {}, arrays [], functions, Date, Map, Set (stored by reference).

let a = 10;            typeof a;   // "number"
let b = "text";        typeof b;   // "string"
let c = null;          typeof c;   // "object" (historic quirk)
let d;                 typeof d;   // "undefined"
let e = [1, 2];        typeof e;   // "object" (use Array.isArray)

2. Function expression

A function expression creates a function inside an expression and assigns it to a variable; the function may be anonymous.

const square = function (n) {      // function expression
  return n * n;
};
console.log(square(5));              // 25

const add = (a, b) => a + b;         // arrow function expression

Differences from a function declaration (function square(n) {...}):

Declaration	Expression
Fully hoisted; can be called before it is defined	Not usable before the assignment line runs (variable hoisted, value is not)
Always has a name	Can be anonymous or named

Uses: callbacks (arr.map(function (x) {...})), IIFEs (function(){ ... })();, closures, and passing functions as values.

3. The this keyword

this refers to the object that is executing the current code; its value depends on how the function is called.

Context	Value of this
Global scope / plain function (non-strict)	window (browser) or globalThis; undefined in strict mode
Method call obj.method()	obj
Constructor new Person()	the newly created object
Event handler	the element that received the event
call, apply, bind	the object explicitly supplied
Arrow function	inherits this from the enclosing scope (no own this)

const user = {
  name: "Raiyan",
  greet() { console.log("Hi, " + this.name); }   // this = user
};
user.greet();                 // Hi, Raiyan

const fn = user.greet;
fn();                          // this lost: undefined / window.name

fn.call({ name: "Ali" });      // Hi, Ali (explicit binding)

const timer = {
  n: 0,
  start() { setInterval(() => this.n++, 1000); }   // arrow keeps this = timer
};