How exactly does <script defer="defer"> work?
I have a few <script> elements, and the code in some of them depend on code in other <script> elements. I saw the defer attribute can come in handy here as it allows code blocks to be postponed in execution.
To test it I executed this on Chrome: http://jsfiddle.net/xXZMN/.
<script defer="defer">alert(2);</script>
<script>alert(1)</script>
<script defer="defer">alert(3);</script>
However, it alerts 2 - 1 - 3. Why doesn't it alert 1 - 2 - 3?
Answer
A few snippets from the HTML5 spec: http://w3c.github.io/html/semantics-scripting.html#element-attrdef-script-async
The defer and async attributes must not be specified if the src attribute is not present.
There are three possible modes that can be selected using these attributes [async and defer]. If the async attribute is present, then the script will be executed asynchronously, as soon as it is available. If the async attribute is not present but the defer attribute is present, then the script is executed when the page has finished parsing. If neither attribute is present, then the script is fetched and executed immediately, before the user agent continues parsing the page.
The exact processing details for these attributes are, for mostly historical reasons, somewhat non-trivial, involving a number of aspects of HTML. The implementation requirements are therefore by necessity scattered throughout the specification. The algorithms below (in this section) describe the core of this processing, but these algorithms reference and are referenced by the parsing rules for script start and end tags in HTML, in foreign content, and in XML, the rules for the document.write() method, the handling of scripting, etc.
If the element has a src attribute, and the element has a defer attribute, and the element has been flagged as "parser-inserted", and the element does not have an async attribute:
The element must be added to the end of the list of scripts that will execute when the document has finished parsing associated with the Document of the parser that created the element.