JavaScript for Loop

In this chapter, we will learn about the JavaScript for loop. The for loop is used to execute a block of code a specified number of times. We will cover:

• What is a for Loop?
• Syntax
• Using the for Loop
• Iterating Over Arrays
• Iterating Over Objects
• Nested for Loops
• Breaking and Continuing in Loops
• Simple Programs using for Loops

What is a for Loop?

The for loop is a control flow statement that allows code to be executed repeatedly based on a given boolean condition. It is particularly useful for iterating over arrays and other collections.

Syntax

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

• initialization: Initializes one or more loop counters and is executed only once.
• condition: Evaluated before each iteration. If true, the loop continues. If false, the loop ends.
• increment: Updates the loop counter after each iteration.

Using the for Loop

Example

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

Output:

0 1 2 3 4 

In the example above, the loop starts with i = 0, increments i by 1 after each iteration, and stops when i reaches 5.

Iterating Over Arrays

The for loop is often used to iterate over arrays.

Example

let fruits = ["apple", "banana", "mango", "orange"]; for (let i = 0; i < fruits.length; i++) { console.log(fruits[i]); } 

Output:

apple banana mango orange 

In the example above, the loop iterates over the fruits array and prints each element.

Iterating Over Objects

To iterate over the properties of an object, you can use a for...in loop.

Example

let person = { firstName: "Ramesh", lastName: "Fadatare", age: 30 }; for (let key in person) { console.log(key + ": " + person[key]); } 

Output:

firstName: Ramesh lastName: Fadatare age: 30 

In the example above, the for...in loop iterates over the properties of the person object and prints each key-value pair.

Nested for Loops

You can use nested for loops to handle more complex scenarios, such as iterating over multi-dimensional arrays.

Example

let matrix = [ [1, 2, 3], [4, 5, 6], [7, 8, 9] ]; for (let i = 0; i < matrix.length; i++) { for (let j = 0; j < matrix[i].length; j++) { console.log(matrix[i][j]); } } 

Output:

1 2 3 4 5 6 7 8 9 

In the example above, the outer loop iterates over the rows of the matrix, while the inner loop iterates over the columns.

Breaking and Continuing in Loops

The break statement is used to exit a loop early, while the continue statement is used to skip the current iteration and continue with the next iteration.

Example with break

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

Output:

0 1 2 3 4 

Example with continue

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

Output:

0 1 2 3 4 6 7 8 9 

Simple Programs using for Loops

Program 1: Print Numbers from 1 to 10

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

Output:

1 2 3 4 5 6 7 8 9 10 

Program 2: Calculate the Sum of the First 10 Natural Numbers

let sum = 0; for (let i = 1; i <= 10; i++) { sum += i; } console.log("Sum:", sum); 

Output:

Sum: 55 

Program 3: Print the Elements of an Array in Reverse Order

let numbers = [1, 2, 3, 4, 5]; for (let i = numbers.length - 1; i >= 0; i--) { console.log(numbers[i]); } 

Output:

5 4 3 2 1 

Program 4: Multiplication Table for a Given Number

let number = 5; for (let i = 1; i <= 10; i++) { console.log(number + " * " + i + " = " + number * i); } 

Output:

5 * 1 = 5 5 * 2 = 10 5 * 3 = 15 5 * 4 = 20 5 * 5 = 25 5 * 6 = 30 5 * 7 = 35 5 * 8 = 40 5 * 9 = 45 5 * 10 = 50 

Program 5: Find the Factorial of a Number

let number = 5; let factorial = 1; for (let i = 1; i <= number; i++) { factorial *= i; } console.log("Factorial:", factorial); 

Output:

Factorial: 120 

Conclusion

In this chapter, you learned about the JavaScript for loop, including its syntax, how to use it, iterating over arrays and objects, nested for loops, and breaking and continuing in loops. We also covered various use cases with simple programs to demonstrate the usage of for loops. The for loop is used for repeating tasks and iterating over collections.