JavaScript for loop

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In JavaScript, loops are essential when you need to perform repetitive tasks. One of the most common types of loops is the for loop, which allows you to repeat a block of code a specified number of times. Whether you’re working with arrays, numbers, or other data structures, the for loop is a powerful tool for iteration.

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1. What is a for Loop in JavaScript?

A for loop is a control structure that allows you to repeatedly execute a block of code as long as a specified condition is true. It is commonly used for iterating over a collection (like an array) or executing a statement multiple times.

Syntax of the for Loop:

for (initialization; condition; increment/decrement) {
  // Code to be executed each time the loop runs
}

• Initialization: This step runs once, setting up the loop (e.g., declaring the loop counter).
• Condition: Before each iteration, the condition is evaluated. If true, the code inside the loop runs. If false, the loop stops.
• Increment/Decrement: After each iteration, this step updates the loop variable (e.g., increasing or decreasing a counter).

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2. Basic for Loop Example

The most basic form of a for loop is one that runs a set number of times, with the counter variable being incremented after each iteration.

Example 1: Basic for Loop

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

Explanation:

• The loop will run 5 times, starting from i = 0 and ending at i = 4.
• After each iteration, i is incremented by 1.
• The output will be:

  Iteration number: 0
  Iteration number: 1
  Iteration number: 2
  Iteration number: 3
  Iteration number: 4
  

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3. Looping Through Arrays with for

The for loop is often used to iterate through arrays and access each element. The loop continues until it has processed all items in the array.

Example 2: Loop Through an Array

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

for (let i = 0; i < fruits.length; i++) {
  console.log(fruits[i]);
}

Explanation:

• The loop runs from i = 0 to i = fruits.length - 1, ensuring every element in the array is accessed.
• The output will be:

  Apple
  Banana
  Orange
  Mango
  

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4. Using the for Loop with a Range

You can also use the for loop to run a block of code a set number of times, without needing an array. This is useful when you want to loop over a range of numbers.

Example 3: Looping Through a Range of Numbers

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

Explanation:

• The loop runs from i = 1 to i = 10, printing each number.
• The output will be:

  1
  2
  3
  4
  5
  6
  7
  8
  9
  10
  

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5. Decrementing in the for Loop

A for loop can also decrement a variable, running in reverse order. This can be useful when you need to iterate backwards through an array or range of values.

Example 4: Decrementing in a for Loop

for (let i = 10; i >= 1; i--) {
  console.log(i);
}

Explanation:

• The loop starts at 10 and decrements i after each iteration until it reaches 1.
• The output will be:

  10
  9
  8
  7
  6
  5
  4
  3
  2
  1
  

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6. Nested for Loops

You can use a for loop inside another for loop to handle more complex iterations, such as when working with multi-dimensional arrays or when you need to perform operations over two variables.

Example 5: Nested for Loops

for (let i = 1; i <= 3; i++) {
  for (let j = 1; j <= 3; j++) {
    console.log("i = " + i + ", j = " + j);
  }
}

Explanation:

• The outer loop runs 3 times (for i = 1, 2, 3).
• For each iteration of i, the inner loop also runs 3 times (for j = 1, 2, 3).
• The output will be:

  i = 1, j = 1
  i = 1, j = 2
  i = 1, j = 3
  i = 2, j = 1
  i = 2, j = 2
  i = 2, j = 3
  i = 3, j = 1
  i = 3, j = 2
  i = 3, j = 3
  

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7. The for...of Loop (ES6+)

In addition to the traditional for loop, JavaScript introduced a new loop in ES6 called the for...of loop. This loop simplifies iteration over iterable objects such as arrays, strings, maps, and more.

Example 6: Using for...of to Loop Through an Array

let colors = ["Red", "Green", "Blue", "Yellow"];

for (let color of colors) {
  console.log(color);
}

Explanation:

• The for...of loop directly iterates over the values in the array, rather than the index.
• The output will be:

  Red
  Green
  Blue
  Yellow
  

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8. The forEach() Method

JavaScript also provides the forEach() method for arrays, which offers a more concise and functional approach to looping through arrays.

Example 7: Using forEach() to Loop Through an Array

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

numbers.forEach(function(number) {
  console.log(number);
});

Explanation:

• forEach() is a method available on arrays that executes a provided function once for each element in the array.
• The output will be:

  10
  20
  30
  40
  

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9. Conclusion

The for loop is an essential tool in JavaScript, offering a versatile way to iterate over arrays, ranges, and more complex data structures. By understanding the different variations and techniques, such as nested loops and the for...of loop, you can handle a wide range of tasks efficiently.

Here’s a quick summary:

• Basic for loop: Iterate a set number of times.
• Loop through arrays: Use a for loop to access elements in an array.
• Decrementing: Use the for loop to iterate in reverse order.
• Nested loops: Handle complex scenarios where multiple variables are involved.
• for...of loop: An easier way to iterate over values in iterable objects (ES6+).
• forEach(): A modern, functional approach to looping over arrays.

By mastering the for loop, you'll be able to handle most iterative tasks in JavaScript effectively.

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