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You can update your SharePoint site’s UI by using JavaScript. This extensibility option is only available for classic SharePoint experiences. You cannot use this option with modern experiences in SharePoint Online, like with communication sites.
You cannot automate running of a JavaScript code in SharePoint site. The page in which JavaScript is loaded should be browsed manually or through some tool to run the code. Also JavaScript based component is not the right choice for what you are trying to do.
You can change a page’s Page Layout after you have logged in and are editing the page (click the Edit icon or click on the Site Actions dropdown menu and select Edit Page). In the ribbon, click on the Page tab and click the Page Layout dropdown. Select the layout you want and wait for the page to refresh.
How to create a website using CSS and HTML?
In this module, you’ll learn how to: 1 Create a basic web page using HTML 2 Apply styles to page elements using CSS 3 Create themes using CSS 4 Add support for switching between themes using JavaScript 5 Inspect the website using browser developer tools
How to inject custom CSS in SharePoint Online?
The Alternate CSS and script editor web parts are not supported in the Modern SharePoint Online sites. However, if you want to inject a custom CSS to your SharePoint Online sites, you can use the react application customizer to inject CSS. Here are the steps at the high level:
How to allow users to create ” modern ” site pages?
Go to the Settings page, select Pages, and then clear Allow users to create new modern pages. For more info, see Allow users to create modern pages. You can prevent a web from using the “modern” page experience by disabling the web scoped feature with ID B6917CB1-93A0-4B97-A84D-7CF49975D4EC (name = “Site Pages”).
Notice also that you should not take a dependency on the html page structure or on the out-of-the-box CSS style names as these might be adjusted without a notice. The Core.EmbedJavaScript sample add-in adds a status bar message to all pages on a SharePoint site, and removes the new subsite link from the Site Contents page by using JavaScript.