Css Defer Async

[Fortun Bawa]

Keep your scripts right before . Async can be used with scripts located there in a few circumstances (see discussion below). Defer won't make much of a difference for scripts located there because the DOM parsing work has pretty much already been done anyway.

Your HTML will display quicker in older browsers if you keep the scripts at the end of the body right before . So, to preserve the load speed in older browsers, you don't want to put them anywhere else.

If your second script depends upon the first script (e.g. your second script uses the jQuery loaded in the first script), then you can't make them async without additional code to control execution order, but you can make them defer because defer scripts will still be executed in order, just not until after the document has been parsed. If you have that code and you don't need the scripts to run right away, you can make them async or defer.

You could put the scripts in the tag and set them to defer and the loading of the scripts will be deferred until the DOM has been parsed and that will get fast page display in new browsers that support defer, but it won't help you at all in older browsers and it isn't really any faster than just putting the scripts right before which works in all browsers. So, you can see why it's just best to put them right before .

Async is more useful when you really don't care when the script loads and nothing else that is user dependent depends upon that script loading. The most often cited example for using async is an analytics script like Google Analytics that you don't want anything to wait for and it's not urgent to run soon and it stands alone so nothing else depends upon it.

Usually the jQuery library is not a good candidate for async because other scripts depend upon it and you want to install event handlers so your page can start responding to user events and you may need to run some jQuery-based initialization code to establish the initial state of the page. It can be used async, but other scripts will have to be coded to not execute until jQuery is loaded.

Both async and defer scripts begin to download immediately without pausing the parser and both support an optional onload handler to address the common need to perform initialization which depends on the script.

Good practice is to keep all the files in your source folder to load source files fast. You need to download all the script, style, icon, and image-related files and put these files into your project folder.

defer attribute: First the defer attribute will download the script file and then wait for HTML parsing. After the end of the HTML parsing, the script will execute. In other words, it will guarantee all the scripts will execute after the HTML parsing.

async attribute: The async attribute will download the script file and execute without waiting for the end of HTML parsing. In other words, it does not guarantee that all the scripts will execute after the HTML parsing.

The async attribute is useful when the script is not used for DOM manipulation. Sometimes you need a script only for server-side operations or for handling cache or cookies, but not for DOM manipulations.

Correction: I originally stated these were picked up by the preload scanner, they're not, they're picked up by the regular parser. However, preload scanner could pick these up, it just doesn't yet, whereas scripts included by executable code can never be preloaded. Thanks to Yoav Weiss who corrected me in the comments.

All information above means that, anything you provide in your html (javascript, css ) browser will pause DOM construction process. If you are familiar with event loop, there is simple rule how event loop executes tasks:

Default - By default, as soon as the browser sees a script tag it downloads the file and then executes the script file. The script files are executed in the order of their occurrence.

Note:In defer, the js files are executed in the order of their occurrence in the HTML file while in the case of the async attribute the script files are executed in the order of download time.

For those who are in Nextjs and using Theme provided by the nextjs, the problem could occur if you are using theme Provider component outside the body tag.Simply, enslose the theme provider component inside the body tag to get the problem fixed:

The HTML element is used to embed executable code or data; this is typically used to embed or refer to JavaScript code. The element can also be used with other languages, such as WebGL's GLSL shader programming language and JSON.

This attribute allows the elimination of parser-blocking JavaScript where the browser would have to load and evaluate scripts before continuing to parse. defer has a similar effect in this case.

Specifies that you want the browser to send an Attribution-Reporting-Eligible header along with the script resource request. On the server-side this is used to trigger sending an Attribution-Reporting-Register-Source or Attribution-Reporting-Register-Trigger header in the response, to register a JavaScript-based attribution source or attribution trigger, respectively. Which response header should be sent back depends on the value of the Attribution-Reporting-Eligible header that triggered the registration.

Note: Alternatively, JavaScript-based attribution sources or triggers can be registered by sending a fetch() request containing the attributionReporting option (either set directly on the fetch() call or on a Request object passed into the fetch() call), or by sending an XMLHttpRequest with setAttributionReporting() invoked on the request object.

Note: Specifying multiple URLs means that multiple attribution sources can be registered on the same feature. You might for example have different campaigns that you are trying to measure the success of, which involve generating different reports on different data.

This attribute explicitly indicates that certain operations should be blocked on the fetching of the script. The operations that are to be blocked must be a space-separated list of blocking tokens listed below.

Normal script elements pass minimal information to the window.onerror for scripts which do not pass the standard CORS checks. To allow error logging for sites which use a separate domain for static media, use this attribute. See CORS settings attributes for a more descriptive explanation of its valid arguments.

This attribute allows the elimination of parser-blocking JavaScript where the browser would have to load and evaluate scripts before continuing to parse. async has a similar effect in this case.

This attribute contains inline metadata that a user agent can use to verify that a fetched resource has been delivered without unexpected manipulation. The attribute must not specified when the src attribute is not specified. See Subresource Integrity.

A cryptographic nonce (number used once) to allow scripts in a script-src Content-Security-Policy. The server must generate a unique nonce value each time it transmits a policy. It is critical to provide a nonce that cannot be guessed as bypassing a resource's policy is otherwise trivial.

Note: An empty string value ("") is both the default value, and a fallback value if referrerpolicy is not supported. If referrerpolicy is not explicitly specified on the element, it will adopt a higher-level referrer policy, i.e. one set on the whole document or domain. If a higher-level policy is not available, the empty string is treated as being equivalent to strict-origin-when-cross-origin.

Indicates that the script is a "classic script", containing JavaScript code. Authors are encouraged to omit the attribute if the script refers to JavaScript code rather than specify a MIME type. JavaScript MIME types are listed in the IANA media types specification.

This value indicates that the body of the element contains an import map. The import map is a JSON object that developers can use to control how the browser resolves module specifiers when importing JavaScript modules.

This value causes the code to be treated as a JavaScript module. The processing of the script contents is deferred. The charset and defer attributes have no effect. For information on using module, see our JavaScript modules guide. Unlike classic scripts, module scripts require the use of the CORS protocol for cross-origin fetching.

This value indicates that the body of the element contains speculation rules. Speculation rules take the form of a JSON object that determine what resources should be prefetched or prerendered by the browser. This is part of the Speculation Rules API.

The embedded content is treated as a data block, and won't be processed by the browser. Developers must use a valid MIME type that is not a JavaScript MIME type to denote data blocks. All of the other attributes will be ignored, including the src attribute.

If present, its value must be an ASCII case-insensitive match for "utf-8". 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.

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, defer or type="module" attributes, as well as inline scripts without the type="module" attribute, are fetched and executed immediately before the browser continues to parse the page.

Scripts loaded using the async attribute will download the script without blocking the page while the script is being fetched. However, once the download is complete, the script will execute, which blocks the page from rendering. This means that the rest of the content on the web page is prevented from being processed and displayed to the user until the script finishes executing. You get no guarantee that scripts will run in any specific order. It is best to use async when the scripts in the page run independently from each other and depend on no other script on the page.

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