JavaScript Loops

What is a Loop?

A loop is used when you want to repeat a task multiple times instead of writing the same code again and again.

for Loop

Syntax

for (initialization; condition; modification) {
    // repetitive task
}

Breakdown

• Initialization → start value
• Condition → how long the loop runs
• Modification → update each step

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Example: Print 0 to 9

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

How it works:

• Start at i = 0
• Run until i < 10
• Increase i by 1 each time

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while Loop

Syntax

initialization;

while (condition) {
    // code
    // increment/decrement
}

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Example: Print 1 to 9

let count = 1;

while (count < 10) {
    console.log(count);
    count++;
}

Key Idea:

• Condition is checked before running the loop

break Statement

What is break?

break is used to stop the loop immediately, even if the condition is still true.

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Example:

for (let i = 1; i <= 10; i++) {
    if (i === 5) {
        break;
    }
    console.log(i);
}

Output:

1
2
3
4

Key Idea:

• Loop stops when i === 5

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continue Statement

What is continue?

continue is used to skip the current iteration and move to the next one.

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Example:

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

Output:

1
2
4
5

do...while Loop

Syntax

do {
    // code
} while (condition);

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Example:

let i = 0;

do {
    console.log('value of i', i);
    i++;
} while (i < 5);

Key Idea:

• Runs at least once, even if condition is false

Print Even Numbers (2 → 10)

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

Logic:

• Start at 2
• Increase by 2 → only even numbers

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Sum from 1 to 5

let sum = 0;

for (let i = 1; i <= 5; i++) {
    sum = sum + i;
}

console.log(sum);

Step-by-step:

sum = 0 + 1 = 1
sum = 1 + 2 = 3
sum = 3 + 3 = 6
sum = 6 + 4 = 10
sum = 10 + 5 = 15

Final Output: 15

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Multiplication Table

const number = 3;

for (let i = 1; i <= 10; i++) {
    console.log(number, 'X', i, '=', i * number);
}

Output:

3 X 1 = 3
3 X 2 = 6
...
3 X 10 = 30

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Quick Comparison

Loop Type	When to Use
for	When you know how many times to run
while	When condition depends on something dynamic
do...while	When code must run at least once

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Key Takeaways

• Loops = repeat tasks efficiently
• for = most commonly used
• Always update variable → avoid infinite loops
• Think in start → condition → update