Serialize String JS Fundamentals

function serializeString(arr) {
    let str = arr[0];
    let charPositions = {};

    // Iterate over the characters in the string and keep track of their positions
    for (let i = 0; i < str.length; i++) {
        let char = str[i];
        if (!charPositions[char]) {
            charPositions[char] = [i];
        } else {
            charPositions[char].push(i);
        }
    }

    // Iterate over the charPositions object and format the output string
    let output = '';
    for (let char in charPositions) {
        let positions = charPositions[char];
        output += `${char}:${positions.join('/')}\n`;
    }
    console.log(output);
}