responsiveText返回“未定义”值

I am using JavaScript Ajax to get response from PHP. In PHP I used echo to return the value for responsiveText in Ajax however on console.log I see response as undefined and therefore further operations are not possible.

AJAX request is:

var xmlhttp = new XMLHttpRequest();
var senddata = "c=" + document.getElementById("image_value").value; 
xmlhttp.open("POST", "image_index.php",true);

xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

xmlhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
    console.log(xmlhttp.responsiveText);    
    }   
};
xmlhttp.send(senddata);

PHP file is:

<?php
$im[] = "value1";
$im[] = "value2";
$im[] = "value3";

$i=-1;
if(isset($_POST['i_value'])){
    $i=$_POST['i_value'];
}
if($i==-1){
    $u="NOTHING";
}
else{
    $u=$im[$i];
}
echo "$u";
?>

where im is an array

Don't fret, it's just a simple typo. The response text is responseText not responsiveText.

I would also recommend using camel case for your variables, for good programming practice. (e.g. dataToSend instead of senddata) In addition, since you used this inside xmlhttp.onreadystatechange I would recommend using this to access its responseText property. See code sample to see what I mean.

var xmlhttp = new XMLHttpRequest();
var dataToSend = "c=" + document.getElementById("image_value").value; 
xmlhttp.open("POST", "image_index.php",true);

xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

xmlhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    console.log(this.responseText);    
  }   
};
xmlhttp.send(dataToSend);