CHAPTER 6 · ITERATION

JavaScript Loops

Loops same type ka code repeat karne ke liye use hote hain. Is chapter me for, while, do...while, break aur continue ko simple examples se samjhenge.

English + Hinglish30 min readPractice included

What is a loop?

A loop repeats a block of code while a rule or condition allows it.

repeat.jsJS

for (let i = 1; i <= 3; i++) {
  console.log("Practice");
}

Hinglish Explanation

Teen baar same line likhne ke bajay loop ko rule de do. Loop repeat ka kaam handle karega.

for loop

for loop tab useful hai jab repetition counter se control ho.

for.jsJS

for (let i = 1; i <= 5; i++) {
  console.log(i);
}

Isme initialization, condition aur update teen important parts hote hain.

for loop ke 3 parts

let i = 1 — starting value.
i <= 5 — loop continue hone ki condition.
i++ — har iteration ke baad update.

while loop

while condition true rehne tak repeat karta hai.

while.jsJS

let count = 1;
while (count <= 3) {
  console.log(count);
  count++;
}

Important: Counter ya condition update karna mat bhoolo, warna loop stop nahi hoga.

do...while loop

do...while body ko kam se kam ek baar run karta hai.

do-while.jsJS

let value = 1;
do {
  console.log(value);
  value++;
} while (value <= 3);

break

break nearest loop ko turant stop karta hai.

break.jsJS

for (let i = 1; i <= 10; i++) {
  if (i === 4) break;
  console.log(i);
}

continue

continue current iteration ka remaining code skip karke next iteration par chala jata hai.

continue.jsJS

for (let i = 1; i <= 5; i++) {
  if (i === 3) continue;
  console.log(i);
}

Even numbers example

even.jsJS

for (let i = 1; i <= 10; i++) {
  if (i % 2 === 0) console.log(i);
}

Running total

sum.jsJS

let total = 0;
for (let i = 1; i <= 5; i++) {
  total += i;
}
console.log(total); // 15

Accumulator

total har round me nayi value add karta hai. Ye pattern totals aur counters me bahut common hai.

Nested loops

Ek loop ke andar doosra loop bhi ho sakta hai.

nested.jsJS

for (let row = 1; row <= 2; row++) {
  for (let col = 1; col <= 3; col++) {
    console.log(row, col);
  }
}

Common loop errors

Off-by-one errors tab hote hain jab loop ek baar extra ya ek baar kam chal jaye. < aur <= ko carefully choose karo.

boundary.jsJS

for (let i = 0; i < 5; i++) {
  console.log(i); // 0 to 4
}

Beginner best practices

Loop condition simple rakho.
Counter ko clearly update karo.
Boundary values test karo.
break aur continue intentional use karo.
Deep nested loops avoid karo when possible.

Common beginner mistakes

Counter update bhool jana.
< vs <= ki wajah se extra iteration chalana.
break aur continue ko same samajhna.
Nested loop me wrong counter update karna.
Loop ke andar unnecessary repeated code chalana.

Chapter checklist

for loop ke 3 parts samajh aaye?
while aur do...while ka difference clear hai?
break aur continue use kar sakte ho?
Even numbers aur running total ka loop bana sakte ho?
Boundary mistake identify kar sakte ho?

Practice Task — Loop Lab

Browser Console me ye exercises khud type karo.

1 se 10 print karo.
10 se 1 countdown karo.
1 se 20 tak even numbers print karo.
1 se 5 ka total calculate karo.
while se 1 se 5 print karo.
do...while ka simple example banao.
break se loop ko 5 par stop karo.
continue se 3 skip karo.
2 × 3 nested-loop output banao.
Ek boundary mistake banao aur phir fix karo.