<script>

O elemento HTML <script> é usado para incluir ou referenciar um script executável.

Esse elemento inclui os atributos globais.

async HTML5

Um atributo booleano indicando que o navegador deve, se possível, executar o script assíncronamente.

Esse atributo não deve ser utilizado se o atributo src estiver ausente (ex. scripts embutidos). Se incluído, nesse caso, ele não terá nenhum efeito.

Scripts inseridos dinamicamente (usando document.createElement) são executados assincronamente por padrão, então para turna-lo uma execução síncrona (ex. executar scripts na ordem que eles foram carregados) atribua async=false.

Veja Browser compatibility para notas no suporte do navegador. Veja também  Scripts assíncronos para asm.js.

crossorigin

Elementos script passam o mínimo de informação para  window.onerror em scripts que não passem na checagem do CORS. Para permitir logs de erro para sites que usem domínimos diferentes para arquivos estáticos, use esse atributo. Veja CORS settings attributes par uma explicação mais detalhada dos argumentos válidos.

defer

Esse atributo Boleano é usado para indicar ao navegador que o script deve ser executado depois que o documento tenha sido parseado, mas antes de disparar o evento DOMContentLoaded (en-US)

Scripts com o atributo defer vão impedir que o evento DOMContentLoaded seja disparado até que o script seja carregado e tenha terminado de ser avaliado.

Esse atributo não deve ser usado se o atibuto src estiver ausente (ex. scripts inline), nesse caso ele não vai ter efeito.

Para conseguir um efeito similar para scripts inseridos dinamicamente use async=false. Scripts com o atributo defer vão ser executados na ordem em que aparecem no document.

integrity

Contains inline metadata that a user agent can use to verify that a fetched resource has been delivered free of unexpected manipulation. See Subresource Integrity.

nomodule

This Boolean attribute is set to indicate that the script should not be executed in browsers that support ES6 modules — in effect, this can be used to serve fallback scripts to older browsers that do not support modular JavaScript code.

src

This attribute specifies the URI of an external script; this can be used as an alternative to embedding a script directly within a document. If a script element has a src attribute specified, it should not have a script embedded inside its tags.

text

Like the textContent attribute, this attribute sets the text content of the element.  Unlike the textContent attribute, however, this attribute is evaluated as executable code after the node is inserted into the DOM.

type

Indicates the type of script represented. The value of this attribute will be in one of the following categories:

• Omitted or a JavaScript MIME type: For HTML5-complient browsers this indicates the script is JavaScript. HTML5 spec urges authors to omit the attribute rather than provided a redundant MIME type. In earlier browsers, this identified the scripting language of the embedded or imported (via the src attribute) code. JavaScript MIME types are listed in the specification.
• module: HTML5 For HTML5-complient browsers the code is treated as a JavaScript module. Processing of the script contents are not affected by the charset and defer attributes. For information on using module, see ES6 in Depth: Modules. 
• Any other value or MIME type: Embedded content is treated as a data block which won't be processed by the browser. The src attribute will be ignored.

Note that in Firefox you can use advanced features such as let statements and other features in later JS versions, by using type=application/javascript;version=1.8 . Beware, however, that as this is a non-standard feature, this will most likely break support for other browsers, in particular Chromium-based browsers.

For how to include exotic programming languages, read about Rosetta.

Atributos obsoletos

charset

If present, its value must be an ASCII case-insensitive match for "utf-8". Both it’s unnecessary to specify the charset attribute, because documents must use UTF-8, and the script element inherits its character encoding from the document.

language

Like the type attribute, this attribute identifies the scripting language in use. Unlike the type attribute, however, this attribute’s possible values were never standardized. The type attribute should be used instead.

Scripts without async or defer attributes, as well as inline scripts, are fetched and executed immediately, before the browser continues to parse the page.

The script should be served with the text/javascript MIME type, but browsers are lenient and only block them if the script is served with an image type (image/*), a video type (video/*), an audio (audio/*) type, or text/csv. If the script is blocked, an error (en-US) is sent to the element, if not a success (en-US) event is sent.

<!-- HTML4 and (x)HTML -->
<script type="text/javascript" src="javascript.js"></script>

<!-- HTML5 -->
<script src="javascript.js"></script>

[1] In older browsers that don't support the async attribute, parser-inserted scripts block the parser; script-inserted scripts execute asynchronously in IE and WebKit, but synchronously in Opera and pre-4.0 Firefox. In Firefox 4.0, the async DOM property defaults to true for script-created scripts, so the default behavior matches the behavior of IE and WebKit. To request script-inserted external scripts be executed in the insertion order in browsers where the document.createElement("script").async evaluates to true (such as Firefox 4.0), set .async=false on the scripts you want to maintain order. Never call document.write() from an async script. In Gecko 1.9.2, calling document.write() has an unpredictable effect. In Gecko 2.0, calling document.write() from an async script has no effect (other than printing a warning to the error console).

[2] Starting in Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1), inserting script elements that have been created by calling document.createElement("script") into the DOM no longer enforces execution in insertion order. This change lets Gecko properly abide by the HTML5 specification. To make script-inserted external scripts execute in their insertion order, set .async=false on them.

Also, <script> elements inside <iframe>, <noembed> (en-US) and <noframes> (en-US) elements are now executed, for the same reasons.

[3] In versions prior to Internet Explorer 10 Trident implemented <script> by a proprietary specification. Since version 10 it conforms to the W3C specification.

[4] The crossorigin attribute was implemented in WebKit in WebKit bug 81438.

[5] WebKit bug 148363 tracks WebKit implementation of Subresource Integrity (which includes the integrity attribute).

[6] Desde Gecko 1.9.2 (Firefox 3.6 / Thunderbird 3.1 / Fennec 1.0), o atributo defer is ignorado nos scripts que  nao tenham o atributo src. However, in Gecko 1.9.1 (Firefox 3.5 / Thunderbird 3.0 / SeaMonkey 2.0) even inline scripts are deferred if the defer attribute is set.

• document.currentScript
• Ryan Grove's <script> and <link> node event compatibility chart