console.log(insert('We are doing some exercises.'));
console.log(insert('We are doing some exercises.','JavaScript '));
console.log(insert('We are doing some exercises.','JavaScript ',18));

function insert(str,ch,pos)
{
    var temp = str.split(' ');
    var arr = [];
    for(var i=0 ; i<=temp.length ; i++)
    {
        if(pos == null && ch==null)
        {
            return temp.join(' ');
        }
        else if(pos== null)
        {
           temp[i]=ch;
        }
        arr.push(temp[i]);
        
    }
    return arr.join(' ')
}