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JavaScript AJAX Basics Programming AJAX Parsing JSON Data

Joseph Michelini
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Joseph Michelini
Python Development Techdegree Graduate 18,692 Points
Posted by Joseph Michelini
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Joseph Michelini
Python Development Techdegree Graduate 18,692 Points

Is there a reason the script tag is added to the document head in this video?

I just wanted to make sure there wasn't a different set of rules regarding script tags and AJAX requests. In theory, shouldn't this script tag be added right before the closing body tag?

Thanks!

2 Answers

Steven Parker
Steven Parker
229,644 Points
Steven Parker
Steven Parker
229,644 Points

This particular script starts an asynchronous process that will almost certainly take longer to complete than the page loading, but I agree that since it does eventually manipulate the DOM it should probably be loaded at the end of the page body.

Loading it in the head is probably intended to give it a "head start" (pun intentional :wink:) in attempt to reduce the delay in presenting the data on the page.

Steven Parker
Steven Parker
229,644 Points
Steven Parker
Steven Parker
229,644 Points

If a script contains only library code and does not immediately interact with the HTML, it is safe (and common) to include it in the <head> section.

But any script that does any DOM interaction should be placed last in the <body>, to be sure the rest of the page has loaded before it starts.

Joseph Michelini
seal-mask
.a{fill-rule:evenodd;}techdegree seal-36
Joseph Michelini
Python Development Techdegree Graduate 18,692 Points
Joseph Michelini
.a{fill-rule:evenodd;}techdegree seal-36
Joseph Michelini
Python Development Techdegree Graduate 18,692 Points

Thank you Steven! Forgive me since I'm so new to server requests, but doesn't this request eventually interact with the DOM? Doesn't it insert HTML?

If so, what difference should I look out for between this and the kind of DOM interaction you're referring to?

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function () {
  if (xhr.readyState === 4 && xhr.status === 200) {
    var employees = JSON.parse(xhr.responseText);
    var statusHTML = '<ul class="bulleted">';
    for (var i = 0; i < employees.length; i += 1) {
      if (employees[i].inoffice === true) {
        statusHTML += '<li class="in">';
      } else {
        statusHTML += '<li class="out">';
      }
      statusHTML += employees[i].name;
      statusHTML += "</li>";
    }
    statusHTML += "</ul>";
    document.getElementById("employeeList").innerHTML = statusHTML;
  }
};
xhr.open("GET", "../video6/data/employees.json");
xhr.send();