Get the index of the nth item of a type in a JavaScript array

When working with JavaScript arrays, you might need to find the index of the nth occurrence of a specific value. This is useful for parsing data, finding patterns, or processing structured arrays.

Problem Statement

We need to write a function getIndex() that takes three parameters: an array arr, a value txt (string or number), and a number n. The function should return the index of the nth appearance of txt in arr. If txt doesn't appear n times, return -1.

Using Array.reduce() Method

The reduce() method provides an elegant solution by maintaining a counter and tracking the target index:

const arr = [45, 76, 54, 43, '|', 54, '|', 1, 66, '-', '|', 34, '|', 5, 76];

const getIndex = (arr, txt, n) => {
    const position = arr.reduce((acc, val, ind) => {
        if (val === txt) {
            if (acc.count + 1 === n) {
                acc['index'] = ind;
            }
            acc['count']++;
        }
        return acc;
    }, {
        index: -1,
        count: 0
    });
    return position.index;
};

console.log(getIndex(arr, '|', 3));  // 3rd occurrence of '|'
console.log(getIndex(arr, 54, 2));   // 2nd occurrence of 54
console.log(getIndex(arr, '-', 3));  // 3rd occurrence of '-' (doesn't exist)

10
5
-1

Using a Simple Loop (Alternative)

For better readability, you can use a simple for loop approach:

const getIndexLoop = (arr, txt, n) => {
    let count = 0;
    for (let i = 0; i < arr.length; i++) {
        if (arr[i] === txt) {
            count++;
            if (count === n) {
                return i;
            }
        }
    }
    return -1;
};

// Test the loop version
console.log(getIndexLoop(arr, '|', 2));  // 2nd occurrence of '|'
console.log(getIndexLoop(arr, 76, 1));   // 1st occurrence of 76

6
1

How It Works

Both methods iterate through the array and:

• Check if the current element matches the target value
• Increment a counter when a match is found
• Return the current index when the counter reaches n
• Return -1 if the loop completes without finding n occurrences

Comparison

Conclusion

Use the for loop approach for better performance and readability. The reduce() method works well for functional programming styles but is less intuitive for this specific problem.