Loop

Loops in Javascript

Loop are used to execute give codeblock repetatively & contineously.

In Javascript 3 types of loop are avilable:

1. for loop
2. while loop
3. do-while loop

for loop

The for loop is the most commonly used loop in JavaScript. It iterates over a block of code a specific number of times.

Syntax :

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

Here, In the above syntax

Initialization: It is usually a variable declaration and initialization.

Condition: This is where you specify the condition that must be true for the loop to continue executing. If the condition is false, the loop will terminate.

Expression: This is where you update the value of the loop counter after each iteration.

Example :

for (let i = 0; i < 5; i++) {
  document.write("<br/> Hello Students");
}

Output :

Hello Students
Hello Students
Hello Students
Hello Students
Hello Students

In the above example,let i=0; initializes a variable i with a value of 0. i<5; sets the condition for the loop to continue running, which is that i must be less than 5. i++ increments i by 1 after each iteration of the loop. document.write("<br/> Hello Students"); writes the string "<br/> Hello Students" to the HTML document.

increment

Example :

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

Output :

1
2
3
4

In the above example, let i=1; initializes a variable i with a value of 1. i<5; sets the condition for the loop to continue running, which is that i must be less than 5. i++ increments i by 1 after each iteration of the loop. The value of 'i' is printed to the console. So, it will display the numbers 1 to 5 on the console.

Example :

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

Output :

1
2
3
4
5

In the above example, a 'for' loop is used to count from 1 to 5, and with each iteration, the value of 'i' is printed to the console. So, it will display the numbers 1 to 5 on the console.

Example :

for (let i = 1; i <= 20; i++) {
  if (i % 2 == 0) {
    console.log(i);
  }
}

Output :

2
4
8
10
12
14
16
18
20

In the above example, a 'for' loop is used to iterate from 1 to 20. During each iteration, the code checks if the current value of 'i' is divisible by 2 (which means it's an even number). If 'i' is even, it will be printed to the console. So, this code will display all the even numbers from 1 to 20 on the console.

Example :

for (let i = 1; i <= 20; i++) {
  if (i % 2 != 0) {
    console.log(i);
  }
}

Output :

1
3
5
7
9
11
13
15
17
19

In the above example, a 'for' loop is used to count from 1 to 20. Within each iteration, there's a condition that checks if the current value of'i'is not divisible by 2, effectively identifying odd numbers. If 'i' is an odd number, it gets printed to the console. Consequently, this code will display all the odd numbers between 1 and 20 on the console.

Example :

for (let i = 1; i <= 20; i += 2) {
  console.log(i);
}

Output :

1
3
5
7
9
11
13
15
17
19

In the above example, a 'for' loop is used to start at 1 and increment 'i' by 2 in each iteration, effectively skipping even numbers. During each iteration, the value of 'i' is printed to the console. So, this code will display a sequence of odd numbers starting from 1 and ending at 19 on the console.

Example :

for (let i = 2; i <= 20; i += 2) {
  console.log(i);
}

Output :

2
4
8
10
12
14
16
18
20

In the above example, a 'for' loop begins with 'i' set to2 and increments 'i' by 2in each iteration. This means it focuses on even numbers. During each iteration, the value of 'i' is printed to the console. So, this code will display a sequence of even numbers starting from 2 and ending at 20 on the console.

decrement

Example :

for (let i = 5; i >= 1; i--) {
  console.log(i);
}

Output :

5
4
3
2
1

In the above example, a 'for' loop starts with 'i' set to 5 and counts down to 1 in each iteration. During each iteration, the value of 'i' is printed to the console. So, this code will display a countdown of numbers from 5 to 1 on the console.

while loop and do-while loop

In a while loop, the loop executes as long as the condition is true.

Syntax :

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

Example :

let i = 0;

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

In the above example, this loop initializes i to 0 and continues to execute as long as i is less than 10. On each iteration of the loop, the value of i is printed to the console using console.log(), and then i is incremented by 1 using i++.

You can also use a do-while loop, which is similar to a while loop but guarantees that the loop body is executed at least once.

Syntax :

do {
  // statements to be executed inside the loop
} while (condition);

Example :

let i = 0;

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

In the above example, this loop initializes i to 0 and executes the loop body at least once. On each iteration of the loop, the value of i is printed to the console using console.log(), and then i is incremented by 1 using i++. The loop continues to execute as long as i is less than 10.

Break and Continue Keyword

break

break keyword is used to immediately terminate a loop and resume execution at the next statement after the loop.

Example :

for (let i = 0; i < 10; i++) {
  if (i == 5) {
    break;
  }
  console.log(i);
}

Output :

0
1
2
3
4

In the above example for loop that starts at 0 and iterates up to 9, with i being the loop counter variable.

During each iteration of the loop, it checks if i is equal to 5 using an if statement. If i is equal to 5, then the break statement is executed, causing the loop to terminate immediately and skip the remaining iterations.

If i is not equal to 5, then the loop continues to execute normally and the console.log(i) statement is executed, which prints the current value of i to the console.

continue

continue is used to skip the current iteration of a loop and move on to the next iteration.

Example :

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

In the above example for loop that starts at 0 and iterates up to 9, with i being the loop counter variable.

During each iteration of the loop, it checks if i is divisible by 2 (i.e., if i % 2 === 0) using an if statement. If i is divisible by 2, then the continue statement is executed, causing the current iteration of the loop to be skipped and moving on to the next iteration.

If i is not divisible by 2, then the loop continues to execute normally and the console.log(i) statement is executed, which prints the current value of i to the console.