无法通过react.js更新JSON文件数据

Question

I am trying to update the comments.json by using one of tutorial example of Comment app of React.js.

I have done very few changes in code :

handleCommentSubmit: function(comment){
var comments = this.state.data;
var newComments = comments.concat([comment]);
this.setState({data: newComments});
$.ajax({
      url: 'test.php',
      dataType: 'json',
      type: 'POST',
      data: comment,
      success: function(data) {
        this.setState({data: data});
      }.bind(this),
      error: function(xhr, status, err) {
        console.error(this.props.url, status, err.toString());
      }.bind(this)
    });
  },

Code of Test.php as folllows:

<?php
   $json = $_POST['json'];
   echo $json;

   if (json_decode($json) != null) { /* sanity check */
     $file = fopen('comments.json','w+');
     fwrite($file, $json);
     fclose($file);
   } else {
     echo "error found"; 
   }
?>

Any help will be good for me, because I am very new to JavaScript.