title: fs.readFile() not read the whole files and everytime the read contents missing few parts
body: <p>I just met a strange problem of fs.readFile() in my program. The file is not big(only 500k in size), it is a JSON.stringify(obj) string. I am trying to read it out and pass it to the front-end. 
  While in the callback of fs.readFile("/path/filename", "utf8", function(e,data){}).I only got in-complete string. </p>
  
  <pre><code>    fs.readFile("/path/filename",'utf8', function (err,data) {
              if(err) {
                 throw err;
              } else {
                 let str = JSON.parse(data); // it will throw error by saying "SyntaxError: Unexpected end of JSON input"
              }
          }); 
  </code></pre>
  
  <p>Have you met this problem？　What's the solution to read the whole/complete file?
  PS:  I have manually checked the file contents, it is good and completed JSON string.</p>