节点。js读取文件内容到字符串，结果是“?? ?”

var fs = require('fs');
var myNumber = undefined;

function addOne(callback) {
  fs.readFile('./User2.txt', 'utf8', function doneReading(err, fileContents) {
    myNumber = fileContents.toString();
    callback();
  });
}

function logMyNumber() {
  console.log(myNumber);
}

addOne(logMyNumber);
var fs = require('fs');
var myNumber = undefine