gistfile1.js

function formatSum(sum){
    sum = sum.toString();
    var mod = sum.length > 3 ? sum.length % 3 : 0;
    console.log(mod);
    // There is a bug which prints '147514' as ' 147 514', with a space at the beginning.
    var res = sum.substr(0, mod) + ' ' + sum.substr(mod).replace(/(\d{3})/g, "$1 ");
    return res;
}

// I can't believe there is no standard way to reverse a string.
String.prototype.reverse = function() { return this.toString().split('').reverse().join('')}

function formatSum2(sum) {
    return sum.toString().reverse().replace(/(\d{3}(?!$))/g, "$1 ").reverse();
}

console.log(formatSum2(14751475)); // 14 751 475
console.log(formatSum2(147514754)); // 147 514 754
console.log(formatSum2(1475147547)); // 1 475 147 547