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JavaScript is a programming language that works on the client side and with which websites can be more functional.

Incorporation in HTML code

JavaScript code can be embedded in HTML pages, so that they acquire functionality. There are several options. May be between labels, it can be contained in an external file, it can be a parameter to HTML tags, and it can be embedded in the form of HTML links. When inserting it into the HTML code, make sure the script is error-free. In this way, mistakes can be avoided, and these can negatively affect search engine optimization.


A scripting language like JavaScript should implement the DOM. DOM stands for "Document Object Model" and is a W3C recommendation. Is a general model for objects in a document.

Possible apps

JavaScript has many different apps. Site visits can be counted, monthly calendars displayed, or form entries verified. Another commonly used example is the calculator. The virtual cart in webshops is another very good application. Bad examples are pop-ups that typically flood users with commercial promotion. It is also feasible to hide the source code of a page from your visitors with JavaScript or prevent the copying of images or texts. Since in this circumstance the right button of the mouse is blocked, the user will also lose other functions and inadvertently would be very limited.

CommonJS can serve as a JavaScript duration. This makes programming interfaces possible, which are suitable for implementation in apps such as databases, web services, and desktop apps.

Consequences of deactivation

Browsers offer users the option to disable JavaScript through NoScript plugins. Visitors to a website can no longer use JavaScript apps, but they will be protected, for example, against annoying commercial promotion. If too many items in a web portal use JavaScript, users will not enjoy visiting that web. Accessibility will be limited. A web portal must be navigable even when JavaScript is disabled.

Security through the Sandbox principle

The Sandbox principle has been developed to protect users from abuse of JavaScript apps. The Sandbox is a kind of cage that restricts language in its scope. An application cannot access local files, for example. In this way, unauthorized persons cannot read or write files.

Relevance for SEO

A search engine generally does not take JavaScript code into account. The interaction enabled for users by JavaScript apps cannot be used by a crawler. It visits a website in the same way that a user who has disabled JavaScript would and cannot access web pages that can only be accessed through JavaScript. Text that is displayed with JavaScript is also not perceived by the search engine in most cases. Redirects are best implemented on the server side.

File size and performance

If a website has long interaction items that have been implemented using JavaScript and placed at the end of the source code, the file size of each individual subpage will increase. This in turn increases the crawling effort for search engines. Loading time for users with slow Internet connections (such as smartphones) increases for each subpage.

In this circumstance, the code must be externalized to an external JavaScript file and embedded at the end of the source code. The external file must also be cached in the user's browser in order to be loaded with the first page view. This results in better performance with the second page view and the search engines will also have an easier task, because the source code of the subpages is "poorer". The crawler's resources are thus used in more content on the web and indirectly promote better repositioning.

Loading time and conversion optimization

Some webmasters tend to externalize the JavaScripts of several subpages into individual external files. This can cause a lot of external JavaScript files to be requested while browsing. This can cause an increase in rendering time because browsers have a set limit of maximum parallel downloads from these external files (this includes images and CSS files). With too many files and a slow Internet connection, it can take a long time for the page to load, which equates to a poor "user experience" and thus can have a negative effect on the conversion rate.

Ideally, all JavaScript files should be merged into a single file that the user loads once (cache), thereby preventing double performance.

Rendering time and conversion optimization

Another trick to boost the performance and conversion of the web is to put the external JavaScript files at the end of the source code. The reason is that browsers may partially stop rendering the web page at the point where an external file was linked and load it first. If the external JavaScript file is on an external server, which responds slowly, the user initially only sees a blank page.

Only once all the scripts and CSS files are loaded will the rendering finish. If you put the reference to the external file at the end of the source code, the content of the HTML file will be displayed and the texts will be visible before the full load. The advantage is that users only have to wait to access the interaction functions of JavaScript apps if there is a slow external server.

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