Efficient Script Loading Async | Defer

Efficient Script Loading Async | Defer

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Anil Verma
·Jan 9, 2021·

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There are three ways of using The Script tag in the HTML document

Regular

Using Async

Using Defer

In a real/live project, the Performance of any application matters a lot. So efficient script loading is also a good factor for a performant application. So I will discuss all three ways of script use and will conclude with the Best Way of using the script in a document.

Regular — In a Regular way, Mostly we use the script tag in the Body section at last. Because if we use it in the Head section, then HTML parsing will stop once the script tag comes in the picture, and once the whole script gets executed then HTML Parsing will resume. So the time taken for DomContentLoaded Event will higher. The problem with this approach is, it does not make par­al­lel requests to fetch the indi­vid­ual files. So the time taken for DomContentLoaded Event will higher.

Using Async/Defer -Makes par­al­lel requests to fetch the indi­vid­ual files. So the time taken for DomContentLoaded Event will less than the Regular way. So, using the script with async/defer attribute is a good idea. But there are use cases and differences for using async/defer attribute.

Similarities between Async and Defer approach

Makes par­al­lel requests to fetch the indi­vid­ual files.

Differences between Async and Defer approach

Async- Scripts are fetched asynchronously and executed immediately.

Defer- Scripts are fetched asynchronously and executed after the whole HTML parsing.

Async- DomContentLoaded Event will not wait for async scripts execution

Defer- DomContentLoaded Event will wait for defer scripts execution

Async- Scripts Execution order is not Guaranteed

Defer- Scripts Execution order is Guaranteed

Async- Mostly used when order is not important like 3rd party libs.

Defer- It is the best solution, Mostly used when execution order matters.

 
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