findFirstUniqueCharacter.js

function findFirstUniqueCharacter( inputString ) {
    let freqCounter = [];

    // The issue using Map data structure is during the retrival. As it does not gaurantee the keys will be retrived in the same order as they were inserted
    // Hence, we use an array of frequency counter. But in this array keys are found using the ascii values of the character.

    inputString.split('').forEach(ch => {
        if (!freqCounter[ch])
            freqCounter[ch] = 1;
        else
            freqCounter[ch]++;
    });

    // Observe this array. It's kinda Map only.
    console.log(freqCounter);

    for (let i = 0; i < inputString.length - 1; i++) {
        let ch = inputString[i];
        if (freqCounter[ch] == 1)
            return ch;
    }
    return 'No Unique Character Found';
}

console.log(findFirstUniqueCharacter('foobar')); // f
console.log(findFirstUniqueCharacter('aabbccdef')); // d
console.log(findFirstUniqueCharacter('aabbcc')); // 'No Unique Character Found'