Loops in JS

 In programming, loops are used when you want to repeat a block of code multiple times. Instead of writing the same code again and again, you can use loops to make your code shorter, cleaner, and more efficient.

In JavaScript, there are several types of loops, but the most commonly used ones are:

• for loop
• while loop
• do...while loop

Let’s understand each one step by step

The for Loop

The for loop is the most commonly used loop when you know how many times you want to run your code.

Syntax:

for (initialization; condition; increment/decrement) {  // code to be executed}

• Initialization: Runs once before the loop starts (usually used to declare a counter variable).
• Condition: Checked before every iteration. If it’s true, the loop runs; if false, the loop stops.
• Increment/Decrement: Updates the counter variable after each iteration.

Example:

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

Output:

Count: 1  
Count: 2  
Count: 3  
Count: 4  
Count: 5

Explanation: The loop starts with i = 1. Each time it runs, i increases by 1 until i <= 5 becomes false.

The for...in Loop

The for...in loop is used to iterate over the properties of an object.

Syntax:

for (variable in object) {  // code to be executed}

Example:

let person = {  name: "Alice",  age: 25,  job: "Designer"}; for (let key in person) {  console.log(key + ": " + person[key]);}

Output:

name: Alice  
age: 25  
job: Designer

Explanation: Each time the loop runs, key takes the name of one property (like "name", "age", etc.), and you can access its value using person[key].

The for...of Loop

The for...of loop is used to iterate over iterable objects like arrays, strings, or sets.

Syntax:

for (variable of iterable) {  // code to be executed}

Example:

let colors = ["red", "green", "blue"]; for (let color of colors) {  console.log(color);}

Output:

red  
green  
blue

Explanation: Here, color takes each element of the array one by one.

The while Loop

The while loop is used when you don’t know in advance how many times you want to run your code. It keeps running as long as the condition remains true.

Syntax:

while (condition) {  // code to be executed}

Example:

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

Output:

Number: 1  
Number: 2  
Number: 3  
Number: 4  
Number: 5

Explanation: The loop checks the condition i <= 5 each time before executing. If it’s true, the code runs; if false, the loop stops. Be careful — if you forget to update i, it can lead to an infinite loop!

Real-world Example:

let answer = ""; while (answer !== "yes" && answer !== "no") {  answer = prompt("Please enter 'yes' or 'no':");}

This loop keeps asking the user for input until they type either "yes" or "no".

The do...while Loop

The do...while loop is very similar to the while loop — but the main difference is that it executes the code at least once, even if the condition is false.

Syntax:

do {  // code to be executed} while (condition);

Example:

let i = 1; do {  console.log("Value:", i);  i++;} while (i <= 5);

Output:

Value: 1  
Value: 2  
Value: 3  
Value: 4  
Value: 5

Explanation: The do block runs first, then the condition is checked. If it’s true, it loops again; if false, it stops. Even if the condition is false from the start, the code still runs once.

Example when the condition is false initially:

let x = 10; do {  console.log("This will run once!");  x++;} while (x < 5);

Output:

This will run once!

Even though x < 5 is false, the message still prints once because of the do...while behavior.

Quick Summary -

Loop Type	When It Runs	Use Case
for	Runs a set number of times	Counting, fixed repetitions
for...in	Loops through object properties	Accessing keys and values in an object
for...of	Loops through iterable elements	Iterating over arrays or strings
while	Runs while condition is true (checked first)	Unknown repetitions until a condition changes
do...while	Runs at least once, checks condition after	Run code once before checking condition