function convertNumberToId (number) {

    var alphabet = 'abcdefghijklmnopqrstuvwxyz'.split('');

​

    if (number < alphabet.length) {

        return alphabet[number];

    } else {

        return (

            convertNumberToId(Math.floor(number / alphabet.length) - 1)

            +

            convertNumberToId(number % alphabet.length)

        );

    }

}

​

function convertIdToNumber(id){

    var alphabet = 'abcdefghijklmnopqrstuvwxyz'.split('');

    if(id.length == 1){

        return alphabet.indexOf(id) >= 0 ? alphabet.indexOf(id) + 1 : -1;

    }

    else if (id.length > 0){

        let t = 0;

        for (let i = 0; i < id.length; i++) {

            t += convertIdToNumber(id.charAt(id.length - 1 - i)) * Math.pow(alphabet.length, i);

        }

        t--;

        return t;

    } else {

        return -1;

    }

}

​

​

function expectConversion(number) {

    var string = convertNumberToId(number),

        actualNumber = convertIdToNumber(string);

​

    if (actualNumber === number) {

        console.log('✔ Expects\t' + number + ' ==> ' + string);

    } else {

        console.warn('❌ Expected\t' + number + ' ==> ' + string + '\tActual\t' + actualNumber + ' <== ' + string);

​

    }

}

​

for(var i = 1; i <= 50; i++){

    expectConversion(i);

}