Javascript’s Async and Defer Attributes: Differences and When to Use Each

Javascript’s async and defer attributes are used to specify the loading behavior of external Javascript files on a webpage. Async tells the browser to execute the script as soon as it is downloaded, while defer tells the browser to wait until the page has finished parsing before executing the script.

TL;DR

In today’s e-commerce universe, website speed is more important than ever. Not only do slow loading times frustrate customers, but they can also have a negative impact on search engine rankings and conversion ratios. One way to improve the speed of your website is by properly using Javascript’s async and defer attributes.

But what exactly are these attributes, and how do they differ? And when should you use each one? Let’s dive in and explore.

The async and defer attributes are used to specify the loading behavior of external Javascript files on a webpage. They both tell the browser to download the script while the page is loading, but they handle execution differently.

The async attribute tells the browser to execute the script as soon as it is downloaded, while the defer attribute tells the browser to wait until the page has finished parsing before executing the script.

In other words, the async attribute allows the script to run concurrently with the rest of the page, while the defer attribute waits until the page has finished loading before running the script.

There are a few key differences between the async and defer attributes:

• Execution order: As mentioned above, the main difference between async and defer is the order in which the scripts are executed. Async scripts run as soon as they are downloaded, while defer scripts wait until the page has finished parsing.
• File size: Async scripts are generally smaller in size than defer scripts, as they do not have to wait for the page to finish parsing before executing. This can lead to faster loading times for async scripts.
• Dependencies: If a script depends on another script to run, it is generally recommended to use the defer attribute. This ensures that the dependent script will not run until the required script has finished loading.

So, when should you use each attribute? Here are a few general guidelines:

• If the script does not have any dependencies and does not need to be executed in a specific order, you can use either async or defer. In this case, it’s generally best to use async, as it allows the script to run concurrently with the rest of the page and can lead to faster loading times.
• If the script has dependencies or needs to be executed in a specific order, you should use the defer attribute. This ensures that the script will not run until the required scripts have finished loading, and that it will execute in the correct order.
• If the script is critical to the page and needs to be executed as soon as possible, you should use the async attribute. This ensures that the script will run as soon as it is downloaded, without waiting for the rest of the page to finish loading.

Impact of webpage speed on an e-commerce store

The speed of a webpage can have a significant impact on an e-commerce store. According to a study by Akamai, a 1-second delay in page load time can result in a 7% reduction in conversions. This means that even small improvements in page speed can lead to significant increases in sales.

Using the async and defer attributes can help improve the speed of your e-commerce store by allowing the browser to download and execute scripts in a more efficient manner. By properly utilizing these attributes, you can help ensure that your store loads quickly and smoothly for your customers.