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layout.js
The file layout.js, when created in a folder, is applied to all the route segments corresponding to the subfolders.
This can be useful if you wish to create a consistent UI sections (eg. header, footer, navagational menu, ads, etc.) across different URLs.
A root layout file must be present at the root of the project, ie. 'app/'. It must contain the <html> and <body> tags;
Layouts in Next.js allow updates on an individual page component only (without having to reload the layout entirely). This is called partial rendering.