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JavaScript AJAX Basics (retiring) AJAX Concepts A Simple AJAX Example

AJAX not working

Here is my code. It is not working. I can click the button and it disappears, but the content from sidebar.html is never displayed.

<!DOCTYPE html>
  <meta charset="utf-8">
  <link href='//fonts.googleapis.com/css?family=Varela+Round' rel='stylesheet' type='text/css'>
  <link rel="stylesheet" href="css/main.css">
  <title>AJAX with JavaScript</title>
     var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function () {
      if (xhr.readystate === 4) {
        document.getElementById('ajax').innerHTML = xhr.responseText;
    xhr.open('GET', 'sidebar.html');
    function sendAJAX() {
      document.getElementById('load').style.display = "none";

  <div class="grid-container centered">
    <div class="grid-100">
      <div class="contained">
        <div class="grid-100">
          <div class="heading">
            <h1>Bring on the AJAX</h1>
            <button id="load" onclick="sendAJAX()">Bring It!</button>
          <ul id="ajax">


I found the issue.

if (xhr.readystate === 4)

needs to be

if (xhr.readyState === 4)