Return the index of first character that appears twice in a string in JavaScript

We are required to write a JavaScript function that takes in a string and returns the index of the first character that appears twice in the string.

If there is no such character then we should return -1.

Syntax

function firstRepeating(str) {
    // Implementation here
}

Example

The code for this will be ?

const str = 'Hello world, how are you';
const firstRepeating = str => {
    const map = new Map();
    for(let i = 0; i < str.length; i++){
        if(map.has(str[i])){
            return map.get(str[i]);
        };
        map.set(str[i], i);
    };
    return -1;
};
console.log(firstRepeating(str));

Output

Following is the output on console ?

2

How It Works

The function uses a Map to track character positions. When it encounters a character for the second time, it returns the index of the first occurrence. In the example, 'l' appears at index 2 and repeats at index 3, so the function returns 2.

Alternative Approach Using indexOf and lastIndexOf

const findFirstRepeating = str => {
    for(let i = 0; i < str.length; i++){
        if(str.indexOf(str[i]) !== str.lastIndexOf(str[i])){
            return i;
        }
    }
    return -1;
};

console.log(findFirstRepeating('Hello world'));
console.log(findFirstRepeating('abcdef')); // No repeating character

2
-1

Comparison

Conclusion

The Map-based approach is more efficient with O(n) time complexity, making it ideal for longer strings. Use indexOf/lastIndexOf for simpler implementations with shorter strings.