When the response is complete, the content is executed using the eval command as shown in this example page. The scripts are executed after they finish downloading. The main drawback of this approach is that the XMLHttpRequest must be served from the same domain as the main page.
The relevant source code from the XHR Eval example follows: [ 11 ]. Using eval is potentially slower than using this mechanism. The relevant source code from the XHR Injection example follows:. Iframes are loaded in parallel with other components in the main page. Whereas iframes are typically used to include one HTML page within another, the Script in Iframe technique leverages them to load JavaScript without blocking, as shown by the Script in Iframe example.
Note that this technique uses A. This is necessary because the iframe expects an HTML document to be returned. All that is needed is to convert the external script to an inline script within an HTML document. Browser cross-site security restrictions prevent JavaScript access from an iframe to a cross-domain parent and vice versa.
One approach is to have the parent reference JavaScript symbols in the iframe via the frames array or document. Iframes also have an innate cost.
This can be done with just a few lines of JavaScript:. Creating a script this way does not block other components during download. As opposed to the previous techniques, Script DOM Element allows you to fetch the JavaScript from a server other than the one used to fetch the main page. The code to implement this technique is short and simple. This is a safe attribute to use when a script does not contain calls to document.
When Internet Explorer downloads the deferred script, it allows other downloads to be done in parallel. The DEFER attribute is an easy way to avoid the bad blocking behavior of scripts with the addition of a single word:. This last technique uses document. This technique, similar to Script Defer, results in parallel script loading in Internet Explorer only. Although it allows multiple scripts to be downloaded in parallel provided all the document.
All of the techniques described in the preceding section improve how JavaScript is downloaded by allowing multiple resources to be downloaded in parallel. But these techniques differ in certain other aspects. Browsers offer multiple browser busy indicators that give the user clues that the page is still loading.
Figure shows four browser busy indicators: the status bar, the progress bar, the tab icon, and the cursor. The status bar shows the URL of the current download. The progress bar moves across the bottom of the window as downloads complete.
The logo spins while downloads are happening. The cursor changes to an hourglass or similar cursor to indicate that the page is busy. The other two browser busy indicators are blocked rendering and blocked onload event. Blocked rendering is very obtrusive to the user experience. The busy indicators that are triggered vary depending on the technique used and the browser being tested. Table shows which busy indicators occur for each of the JavaScript download techniques.
The other techniques have mixed behavior. Abbreviations are as follows: Chr Chrome 1. In some cases, the busy indicators are desirable for a better user experience: they let the user know the page is working. In other situations, it would be better not to show any busy activity, thus encouraging users to start interacting with the page. If a higher-level policy is not available, the empty string is treated as being equivalent to strict-origin-when-cross-origin.
This attribute specifies the URI of an external script; this can be used as an alternative to embedding a script directly within a document. This attribute indicates the type of script represented. The value of this attribute will be in one of the following categories:. Like the type attribute, this attribute identifies the scripting language in use. The type attribute should be used instead. If the script is blocked, an error is sent to the element, if not a load event is sent.
Browsers that support the module value for the type attribute ignore any script with a nomodule attribute. That enables you to use module scripts while also providing nomodule -marked fallback scripts for non-supporting browsers. 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 Firefox.
To request script-inserted external scripts be executed in the insertion order in browsers where the document. Warning: Never call document. In Firefox 3. In Firefox 4, calling document. It is possible to associate an action with a certain number of events that occur when a user interacts with a user agent.
Each of the "intrinsic events" listed above takes a value that is a script. The script is executed whenever the event occurs for that element. When these elements do not appear within a form, they may be used to augment the graphical user interface of the document. For instance, authors may want to include press buttons in their documents that do not submit a form but still communicate with a server when they are activated. The following examples show some possible control and user interface behavior based on intrinsic events.
In the following example, userName is a required text field. When a user attempts to leave the field, the onblur event calls a JavaScript function to confirm that userName has an acceptable value.
Here is a JavaScript example for event binding within a script. First, here's a simple click handler:. Note that "document. The following example illustrates how scripts may modify a document dynamically. The following script:. The following sections discuss how authors may create documents that work for user agents that don't support scripting.
In the following example, a user agent that executes the SCRIPT will include some dynamically created data in the document.
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