function sentenceCase (str) {  
      if ((str===null) || (str===''))  
           return false;  
      else  
       str = str.toString();  
      
     return str.replace(/\w\S*/g, function(txt){return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();});  
    }  
      
    console.log(sentenceCase('PHP exercises. python exercises.'));