title: Cannot read the Ajax response returned by PHP file
body: <p>I am a newbie in Ajax.</p>
  
  <p>I cannot get the correct response from PHP file. The answer in console is "Undefined". When I just address simply to the <strong>Response</strong> variable, the console gives me the Object; it seems that I cannot grab the value of the PHP file result.</p>
  
  <p>Here's the HTML code:</p>
  
  <pre><code>&lt;form id="form1" method="post" onsubmit="return send_form(this)"&gt;
          &lt;p&gt;Number 1= &lt;input type="text" name="number1"&gt;&lt;/p&gt;
          &lt;p&gt;Number 2= &lt;input type="text" name="number2"&gt;&lt;/p&gt;
          &lt;input type="submit" value="calculate"&gt;
  
      &lt;/form&gt;
          &lt;script&gt;
             function send_form(form1){
                      let response=$.ajax("1_1.php",{
                      dataType:'json',
                      data:{n1:form1.number1.value, n2:form1.number2.value},
                      }
                      );
                  console.log(response.c);
  
                  return false;
              }
  
          &lt;/script&gt;
  </code></pre>
  
  <p>and this is my PHP file</p>
  
  <pre><code>&lt;?php
  
  $a=$_GET['n1'];
  $b=$_GET['n2'];
  $c=$a+$b;
  
  echo json_encode(['c'=&gt;$c]);
  
  ?&gt;
  </code></pre>