Should I async or defer JavaScript?

Is defer better than async?

DEFER always causes script execution to happen at the same time as or later than ASYNC. Presumably, scripts are made DEFER or ASYNC because they are less important for the critical content on the page. Therefore, it’s better to use DEFER so that their execution happens outside of the main rendering time.

Should you defer JavaScript?

If you have any noncritical JavaScript file, or any code that depends on the DOM being rendered to run, load it in the head with the defer attribute. This will result in the best performance on most browsers. That is, of course, unless your entire document is less than 14kb minified and gzipped.

What happens when you defer JavaScript?

With defer, visitors’ browsers will still download the scripts while parsing the HTML, but they will wait to parse the script until after the HTML parsing has been completed. … This means that visitors’ browsers will not download or execute any JavaScript until the initial page load is finished.

Can I use async and defer together?

No you can use defer or async, but not both.

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What is the use of defer?

The defer is a Boolean value, used to indicate that script is executed after the document has been parsed. It works only with external scripts (i.e., works only when we are specifying the src attribute in <script> tag).

Why defer is use in script tag?

Scripts with the defer attribute will execute in the order in which they appear in the document. This attribute allows the elimination of parser-blocking JavaScript where the browser would have to load and evaluate scripts before continuing to parse.

Should jQuery be deferred?

Google recommends deferred loading of JavaScript to speed up initial rendering. My page uses jQuery to set up some tabs which are placed low on the page (mostly out of initial view) and I’d like to defer jQuery until AFTER the page has rendered.

When should I load JavaScript?

Because of the fact that browsers have to pause displaying content of a page when it’s parsing a Javascript file, the recommendation is to load the Javascript at the bottom of the page to speed up displaying a page’s content.

What is the purpose of Defer in HTML?

The defer attribute is a boolean attribute. If the defer attribute is set, it specifies that the script is downloaded in parallel to parsing the page, and executed after the page has finished parsing. Note: The defer attribute is only for external scripts (should only be used if the src attribute is present).

Is JavaScript parser blocking?

By default all JavaScript is parser blocking. Because the browser does not know what the script is planning to do on the page, it assumes the worst case scenario and blocks the parser.

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How do you use defer?

verb (used with object), de·ferred, de·fer·ring. to put off (action, consideration, etc.) to a future time: The decision has been deferred by the board until next week. to exempt temporarily from induction into military service.

Does DOMContentLoaded wait for scripts?

External style sheets don’t affect DOM, so DOMContentLoaded does not wait for them. The reason for this is that the script may want to get coordinates and other style-dependent properties of elements, like in the example above. … As DOMContentLoaded waits for scripts, it now waits for styles before them as well.

What is a caption in HTML?

Description. The HTML <caption> tag defines the title of a table in the HTML document. Browsers traditionally render the text found in the <caption> tag above the table, but you can change this behavior with the CSS caption-side property. This tag is also commonly referred to as the <caption> element.