JavaScript标签<script>中的defer和async

<script>

Let’s start by defining what

<script>

without any attributes does. The HTML file will be parsed until the script file is hit, at that point parsing will stop and a request will be made to fetch the file (if
it’s external). The script will then be executed before parsing is resumed.

<script async>

async

downloads the file during HTML parsing and will pause the HTML parser to execute it when it has finished downloading.

<script defer>

defer

downloads the file during HTML parsing and will only execute it after the parser has completed.

defer

scripts are also guarenteed to execute in the order
that they appear in the document.

When should I use what?

Typically you want to use

async

where possible, then

defer

then no attribute. Here are some general rules to follow:

If the script is modular and does not rely on any scripts then use

async

.
If the script relies upon or is relied upon by another script then use

defer

.
If the script is small and is relied upon by an

async

script then use an inline

script

with no attributes placed above the

async

scripts.

Thanks:
http://peter.sh/experiments/asynchronous-and-deferred-javascript-execution-explained/ http://www.growingwiththeweb.com/2014/02/async-vs-defer-attributes.html