Finding reversed index of elements in arrays - JavaScript

We need to create a JavaScript function that finds the reversed index of an element in an array without actually reversing it. This function calculates what position an element would occupy if the array were reversed.

If the element is not present in the array, the function returns -1. Otherwise, it returns the index position the element would have in a reversed array.

Understanding Reversed Index

The reversed index is calculated using the formula: length - originalIndex - 1

For example, in array [45, 74, 34, 32, 23, 65]:

• Element 23 is at index 4
• Array length is 6
• Reversed index = 6 - 4 - 1 = 1

Implementation

const arr = [45, 74, 34, 32, 23, 65];

// Define the reversedIndexOf function
const reversedIndexOf = function(num) {
    const { length } = this;
    const ind = this.indexOf(num);
    
    // Return -1 if element not found
    if (ind === -1) {
        return -1;
    }
    
    // Calculate reversed index
    return length - ind - 1;
};

// Attach function to Array prototype
Array.prototype.reversedIndexOf = reversedIndexOf;

// Test the function
console.log("Array:", arr);
console.log("Reversed index of 23:", arr.reversedIndexOf(23));
console.log("Reversed index of 45:", arr.reversedIndexOf(45));
console.log("Reversed index of 99:", arr.reversedIndexOf(99));

Array: [ 45, 74, 34, 32, 23, 65 ]
Reversed index of 23: 1
Reversed index of 45: 5
Reversed index of 99: -1

How It Works

The function follows these steps:

1. Get the array length using destructuring
2. Find the original index using indexOf()
3. Return -1 if the element doesn't exist
4. Calculate reversed index using the formula length - originalIndex - 1

Testing with Different Values

const testArray = ['apple', 'banana', 'cherry', 'date'];

console.log("Original array:", testArray);
console.log("Reversed index of 'cherry':", testArray.reversedIndexOf('cherry'));
console.log("Reversed index of 'apple':", testArray.reversedIndexOf('apple'));
console.log("Reversed index of 'grape':", testArray.reversedIndexOf('grape'));

Original array: [ 'apple', 'banana', 'cherry', 'date' ]
Reversed index of 'cherry': 1
Reversed index of 'apple': 3
Reversed index of 'grape': -1

Conclusion

The reversedIndexOf function efficiently calculates reversed array positions using mathematical formula rather than actually reversing arrays. This prototype extension provides a useful utility for index calculations.