Reverse an array in different ways.

There is a built-in array method in Javascript that reverses the array i.e. reverse() method.

1. Using reverse() method:

reverse() method: The reverse() method reverses the array in place, meaning it modifies the original array directly and does not return a new array.

Example:

const givenArr = [5, 2, 3, 7, 9, 15, 20];
const reversedArr1 = givenArr.reverse();

// Prints the reversed array
console.log(reversedArr1);  // Output: [20, 15, 9, 7, 3, 2, 5]
console.log(givenArr);  // Output: [20, 15, 9, 7, 3, 2, 5]

• The reverse() method directly modifies the original array by reversing its elements.
• The reversed array is also returned as the result of the method

In the above example, If you want to avoid modifying the original array, you can first create a copy of the array using .slice() and then apply .reverse() to the copy.

const givenArr = [5, 2, 3, 7, 9, 15, 20];

// Create a copy of the array and reverse the copy
const reversedArr = givenArr.slice().reverse();

console.log(reversedArr);  // Output: [20, 15, 9, 7, 3, 2, 5]
console.log(givenArr);   // Output: [5, 2, 3, 7, 9, 15, 20]

2. Using a for-loop:

const givenArr = [5, 12, 3, 7, 21, 9, 45, 90];
const printReversedArr = (arr) => {
    const arrLen = arr.length;
    for(let i = 0; i < arrLen / 2; i++) {
        let temp = arr[i];
        arr[i] = arr[arrLen - 1 - i];
        arr[arrLen - 1 - i] = temp
    }
    return arr ;
}
console.log(printReversedArr(givenArr));  // Output: [90, 45, 9, 21, 7, 3, 12, 5]

3. Using a for-loop (with a new array):

const givenArr = [5, 12, 3, 7, 21, 9, 45, 90];
const getReverseArr = (arr) => {
    let reversedArr = [];
    for(let i = arr.length - 1; i >= 0 ; i --) {
        reversedArr.push(arr[i]);
    }
    return reversedArr;
}
console.log(getReverseArr(givenArr));  // Output: [90, 45, 9, 21, 7, 3, 12, 5]

4. Using forEach() method:

const givenArr = [ 12, 3, 21, 9, 45, 22, 7];
const reverseArray = (arr) => {
    let reversedArr = [];
    arr.forEach(item => {
        reversedArr.unshift(item);
    });
    return reversedArr;
}
console.log(reverseArray(givenArr));  // Output: [7, 22, 45, 9, 21, 3, 12]

5. Using reduce() method:

const givenArr = [ 12, 3, 21, 9, 45, 22, 7];
const reverseArray = (arr) => {
    return arr.reduce((reversed, current) => [current, ...reversed], []);
}
console.log(reverseArray(givenArr));  // Output: [7, 22, 45, 9, 21, 3, 12]

6. Using unshift() method:

const givenArr = [5, 2, 3, 7, 9, 15, 20];
const  reverseArray = (arr) => {
    let reversedArr = [];
    for (let i = 0; i < arr.length; i++) {
        reversedArr.unshift(arr[i]);
    }
    return reversedArr;
}
console.log(reverseArray(givenArr));  // Output: [20, 15, 9, 7, 3, 2, 5]

7. Using a while loop (in-place modification):

const givenArr = [10, 6, 9, 3, 4, 5, 1];
const reverseArray = (arr) => {
    let start = 0;
    let end = arr.length - 1;
    while (start < end) {
        // Swap elements at start and end
        let temp = arr[start];
        arr[start] = arr[end];
        arr[end] = temp;

        start++;
        end--;
    }
    return arr;
}
console.log(reverseArray(givenArr));  // Output: [1, 5, 4, 3, 9, 6, 10]

8. Using recursion:

const givenArr = [10, 6, 9, 3, 4, 5, 1];
const reverseArray = (arr) =>  {
    if (arr.length === 0) {
        return arr;
    } else {
        return reverseArray(arr.slice(1)).concat(arr[0]);
    }
}
console.log(reverseArray(givenArr));  // Output: [1, 5, 4, 3, 9, 6, 10]
console.log("Original Array: ", givenArr);  // Output: Original Array: [10, 6, 9, 3, 4, 5, 1]