Loop in JS

What is Loop?

• A loop is used to execute a block of code repeatedly as long as a specified condition is true. The loop stops when the condition becomes false.
• A loop is used to perform the same task multiple times without writing the same code repeatedly.

Without Loop:

console.log("Welcome");
console.log("Welcome");
console.log("Welcome");
console.log("Welcome");
console.log("Welcome");

With Loop:

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

Welcome
Welcome
Welcome
Welcome
Welcome

Why do we use loop?

• Reduce the lines of code.
• Without loops, we need to write the same code multiple times manually. Using loops, we can execute a block of code repeatedly and automatically until the specified condition becomes false.
• Easier to maintain.
• Used to iterate arrays, objects...

Types of Loop:

There are two types of Loops:

Entry Controlled Loops:

• Any loop where we check the test condition before entering the loop is an entry controlled loop.
• If the condition is false initially, the loop body will not execute even once.

Exit Controlled Loops:

• Any loop where we check the test condition after the statements are executed once is an exit controlled loop.
• The loop body executes at least once, even if the condition is initially false.

             While loop
             do...while loop
             for loop
             for .. in loop
             for .. of loop

While loop:

• While loop is an entry-controlled loop.
• It executes the block of code when the condition is true, when the condition becomes false, the loop stops.

Syntax:

while(condition)
{
//code
}

Example:

// While loop

let n=1;

while(n<=5)
{
    console.log(n);
    n++;
}

Output:

1
2
3
4
5

Here:

1. Check 1 <= 5 → true → print 1
2. Check 2 <= 5 → true → print 2
3. Check 3 <= 5 → true → print 3
4. Check 4 <= 5 → true → print 4
5. Check 5 <= 5 → true → print 5
6. Check 6 <= 5 → false → stop the loop

---

do.. While Loop:

• Do... While is a exit-controlled loop.
• The loop body executes at least once, even if the condition is initially false.
• The condition is checked after executing the loop body.

Syntax:

do
{

//code

}while(condition)

Example:

//do while

let i = 10;

do {
    console.log(i);
    i++;
} while (i <= 5);

Output:

10

Here, i <= 5 is initially false, but 10 is printed because the loop body executes before the condition is checked.

---

For Loop:

• For loop is an entry-controlled loop.
• For loop is used when the number of iteration is known.
• In for loop, initialization, condition, and increment/decrement are written in the same line, separated by semicolons (;).

Syntax:

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

Three Parts of for Loop:

for (let i = 1; i <= 5; i++)
     ↑          ↑       ↑
     |          |       |
Initialization Condition Update

Initialization → Executes only once at the beginning.
Condition → Checked before every iteration.
Increment/Decrement → Updates the value after each iteration.

Example:

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

Output:

1
2
3
4
5

---

for .. in Loop:

• The for...in loop is used to iterate over the keys (property names) of an object.
• In each iteration, the variable contains the key/index, not the value.

Syntax:

for (let key in object) {
    // code
}

Example:

// for in
let Data={
   name:"Raksha",
   location:"Thiruvarur",
   isStudent:true,
   age:21
}

for(let key in Data)
{
    console.log(key);
}

Output:

name
location
isStudent
age

Example:

let animals=["Lion","Tiger","Cat"]


for(let index in animals)
{
    console.log(index);
}

Output:

0
1
2

---

for .. of Loop:

• The for...of loop is used to iterate over the values of an iterable objects.
• It is commonly used with arrays, strings, Sets, and Maps.
• It directly gives you the value, not the key/index.

Syntax:

for (let value of iterable) {
    // code
}

Example:

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

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

Output:

Apple
Banana
Mango

---