console.log(text_truncate('We are doing JS string exercises.'))
console.log(text_truncate('We are doing JS string exercises.',19,'...'))
console.log(text_truncate('We are doing JS string exercises.',15,'!!'))

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