JavaScript Basics — Variables, Types, Operators, Conditions and Loops

JavaScript (JS) makes web pages interactive — menus, forms, live updates. It also runs on servers through Node.js. You can try every example here in the browser console (F12 → Console) or with node file.js.

1. Variables: let and const

const name = "Asha";      // can't be reassigned
let score = 80;           // can change
score += 15;
console.log(name, score);
console.log(`${name} scored ${score} marks`);   // template literal

Asha 95
Asha scored 95 marks

• Use const by default, let when the value must change.
• Avoid var — it ignores block scope and causes surprises.
• Template literals use backticks and ${…}.

2. Data types

console.log(typeof 42, typeof 3.5, typeof "hi", typeof true);
console.log(typeof undefined, typeof null, typeof [1, 2], typeof { a: 1 });
console.log(0.1 + 0.2, 0.1 + 0.2 === 0.3);
console.log(Number("42") + 1, "42" + 1, "5" * 2);

number number string boolean
undefined object object object
0.30000000000000004 false
43 421 10

• Primitives: number, string, boolean, undefined, null, bigint, symbol.
• Everything else is an object (arrays, functions, dates).
• typeof null is "object" — a famous historical bug.
• + with a string joins text; other operators convert to numbers.

3. == vs ===

console.log(5 == "5", 5 === "5");
console.log(0 == false, 0 === false);
console.log(null == undefined, null === undefined);

true false
true false
true false

== converts types before comparing (confusing). Always use === (strict: same type and value) and !==.

4. Truthy and falsy

Falsy values: false, 0, "", null, undefined, NaN. Everything else is truthy — including "0", [] and {}.

const items = [];
console.log(Boolean(""), Boolean("0"), Boolean([]));
const user = null;
console.log(user?.name ?? "Guest");

false true true
Guest

• ?. (optional chaining) stops safely if something is null/undefined.
• ?? (nullish coalescing) gives a default only for null/undefined.

5. Conditions

const marks = 72;
let grade;
if (marks >= 90) grade = "A";
else if (marks >= 60) grade = "B";
else grade = "C";
console.log(grade, marks >= 40 ? "pass" : "fail");

B pass

switch works too (with break, like C and Java).

6. Loops

for (let i = 1; i <= 3; i++) console.log("round", i);

const cities = ["Jaipur", "Pune", "Indore"];
for (const city of cities) console.log(city.toUpperCase());

const marksBy = { maths: 88, dsa: 92 };
for (const subject in marksBy) console.log(subject, marksBy[subject]);

let n = 1;
while (n < 100) n *= 3;
console.log(n);

round 1
round 2
round 3
JAIPUR
PUNE
INDORE
maths 88
dsa 92
243

• for…of → values of an array/string.
• for…in → keys of an object.

Common mistakes

• Using == and getting surprising results.
• Forgetting that "5" + 1 is "51".
• Using var (function-scoped) inside loops and blocks.

Quick recap

• const by default, let when needed, no var.
• Types: number, string, boolean, undefined, null, object…; typeof.
• Always ===; know the falsy values; use ?. and ??.
• for, for…of (values), for…in (keys), while.