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Hello World!

JavaScript programs can be inserted almost anywhere into an HTML document using the <script> tag.

<!DOCTYPE HTML>
<html>
<body>
  <p>Before the script...</p>
  <script>
    alert( 'Hello, world!' );
  </script>
  <p>...After the script.</p>
</body>
</html>

The <script> tag contains JavaScript code which is automatically executed when the browser processes the tag.

HTML <script> defer Attribute​

A script that will be downloaded in parallel to parsing the page, and executed after the page has finished parsing:

<script src="demo_defer.js" defer></script>
info

The defer attribute is only for external scripts (should only be used if the src attribute is present).

There are several ways an external script can be executed:

If async is present: The script is downloaded in parallel to parsing the page, and executed as soon as it is available (before parsing completes)

If defer is present (and not async): The script is downloaded in parallel to parsing the page, and executed after the page has finished parsing

If neither async or defer is present: The script is downloaded and executed immediately, blocking parsing until the script is completed

If src is set, the script content is ignored.

A single <script> tag can’t have both the src attribute and code inside.

This won’t work:

<script src="file.js">
  alert(1); // the content is ignored, because src is set
</script>

We must choose either an external <script src="…"> or a regular <script> with code.

The example above can be split into two scripts to work:

<script src="file.js"></script>
<script>
  alert(1);
</script>