Integration testing focuses on the dependencies between different components of the application. It ensures that these components work together as expected. While interactions and behavior are important aspects, they are not the primary focus of integration testing. Functions are a narrower concept and do not encompass all aspects of integration.

To validate incoming request payloads in Express, it's recommended to use a library like express-validator. This library provides a convenient way to validate and sanitize user input, making it an essential tool for building secure and robust applications.

In Node.js, the global object is a global context object that contains various properties and methods available throughout the application. globalThis is a reference to the global object, which allows cross-environment compatibility. console, module, and require are not properties of the global object but are commonly used in Node.js for various purposes.

The primary difference is that OAuth 1.0 requires client registration, while OAuth 2.0 does not. OAuth 2.0 introduced a more streamlined and flexible approach to authorization. The other options describe differences but not the primary distinction.

In Express.js, the :id? in a route path like "/users/:id?" denotes that id is an optional parameter. This means that the id parameter may or may not be present in the URL, and the route will still match. The other options are not correct in this context (required would mean the parameter is mandatory, query is used for query parameters, and body is used for request bodies).

To serve static files such as images, CSS files, and JavaScript files, a special folder called the public folder is commonly used in Express.js. This folder typically contains all your publicly accessible static assets.