JavaScript Loops

Table of Contents

• Intro
• for Loop
• while Loop
• do...while Loop
• for...of Loop
• for...in Loop
• Loop Control Statements
• Real-Life Example
• Common Mistakes
• When to Use What
• Interview Q&A
• MCQ

JavaScript Loops Tutorial (Complete Guide)

Loops let you repeat a block of code efficiently instead of writing it multiple times. They are essential for iterating arrays, processing data, and automating repetitive work.

1. for Loop (Most Common)

Use for when you know how many times the loop should run.

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

Here, the loop starts at i = 1, runs until i <= 5, and increments each time.

2. while Loop (Condition-Based)

Use while when the number of iterations is not fixed.

let i = 1;

while (i <= 5) {
    console.log("Number:", i);
    i++;
}

3. do...while Loop (Runs at Least Once)

This loop executes at least once, even if condition is false initially.

let i = 1;

do {
    console.log("Number:", i);
    i++;
} while (i <= 5);

4. for...of Loop (Array Values)

Use for...of to loop through values in arrays or iterable objects.

let fruits = ["Apple", "Banana", "Mango"];

for (let fruit of fruits) {
    console.log(fruit);
}

5. for...in Loop (Object Keys)

Use for...in to loop through object properties.

let person = {
    name: "John",
    age: 25
};

for (let key in person) {
    console.log(key + ": " + person[key]);
}

7. Loop Control Statements

break (Stop Loop)

for (let i = 1; i <= 5; i++) {
    if (i === 3) break;
    console.log(i);
}
// Output: 1, 2

continue (Skip Iteration)

for (let i = 1; i <= 5; i++) {
    if (i === 3) continue;
    console.log(i);
}
// Output: 1, 2, 4, 5

8. Real-Life Example

let numbers = [10, 20, 30, 40];
let sum = 0;

for (let num of numbers) {
    sum += num;
}

console.log("Total:", sum);

9. Common Mistakes

• Infinite loop due to missing update step:

let i = 1;
while (i <= 5) {
    console.log(i);
    // missing i++
}

• Using for...in for arrays (not recommended). Use for...of for array values.

10. When to Use What

• Use for when you know the count.
• Use while when condition controls execution.
• Use do...while when at least one run is needed.
• Use for...of for arrays.
• Use for...in for objects.

Related Topics

Control Flow, Switch Statement, and Functions together cover most program flow and iteration patterns.

10 Loops Interview Q&A

10 Loops MCQs