function count_up(obj) {
    var len = obj.value.length;
    $("span#numbers").text(len);
  };

function loadFile() {
  return new Promise((resolve, reject) => {
    try{
        //Function about loading file
        //...
        resolve(obj);
    } catch (e) {
        reject(e);
    }
  });
};

loadFile()  //Will do count_up after the text is loaded
    .then(function(obj){
         count_up(obj);
     })
    .catch((reason) => {
         console.log('Handle rejected promise ('+reason+') here.');
     });