use JavaScript in HTML

the `<script>` element

There are 6 attributes for the <script> element:

async: the script should begin downloading immediately, but should not prevent other actions on the page. valid only for external script files.
- HTML5 introduces async
- async does not guarantee that scripts will be executed in order.
charset: this attribute is rarely used, because most browsers don't honor its value.
defer: the script will be loaded after the page has been completely parsed and displayed.
- the script will start downloading after the browser has received the closing </html> tag.
- the HTML5 specification indicates that scripts will be executed in the order in which they appear.
- defer attribute may not be fully supported by all browsers. Because of this, it's still best to put deferred scripts at the bottom of the page.
language: deprecated. do not use.
src
type: indicates the content type (also MIME type). this value is safe to omit, as "text/javascript" is assumed when missing.

<noscript>使得不支持javascript的浏览器可以平稳退化，用以显示替代内容。包含在<noscript>元素中的内容只有在下列情况下才会显示出来：

as with inline javascript code, processing of the page is halted while the external file is interpreted.
always use <script src="source.js"></script> with open and close tag. It's not legal to have self-closing tag <script /> in HTML.
browsers don't check the file extension of included JavaScript files. This leave open the possibility of dynamically generating JavaScript code using JSP or other server-side scripting language. If you don't use a .js extension, make sure that your server is returning the correct MIME type.
<script> allows to include JavaScript files from outside domains. Make sure you know where the code comes from.
put <script> at the end of <body> tag.
- rendering begins when the browser receives the opening <body> tag
- it won't block the page rendering. The page is completely rendered in the browser before the javascript code is processed.