Separate a number with commas

separate.js

function partition(array, size) {
    if (array.length === 0) return [];
    const part = array.slice(0, size);
    return [part].concat(partition(array.slice(part.length), size));
}

function separate(number, separator = ',') {
    const parts = number.toString().split('.');
    
    parts[0] = partition(parts[0].split('').reverse(), 3)
        .map(arr => arr.reverse().join(''))
        .reverse()
        .join(separator);
        
    return parts.join('.');
}

// Examples

console.log(separate(11243461335.12423));
// => 11,243,461,335.12423
console.log(separate(11243461335));
// => 11,243,461,335
console.log(separate(10));
// => 10
console.log(separate(100));
// => 100
console.log(separate(1000));
// => 1,000