The primary purpose of performance optimization in a Node.js application is to enhance application speed. While other factors like memory usage and code readability are important, the primary goal is to make the application run faster and respond more efficiently to user requests.

The async keyword is used before a function to make it return a Promise. It allows you to work with asynchronous code in a more synchronous-like fashion by enabling the use of await within the function.

The primary purpose of running npm init in a Node.js project is to create a new Node.js project. This command initializes a package.json file, which is essential for managing project dependencies and configurations. It does not install or uninstall Node.js itself.

To correctly implement a middleware function in Express.js, you define a function with the signature (req, res). This function can also take an optional next parameter if you want to pass control to the next middleware in the stack. The (req, res) signature allows you to access and modify the request and response objects.

The main difference is that SQL databases use a structured, tabular format for data storage, adhering to a fixed schema, while NoSQL databases use various flexible data models, making them suitable for unstructured or semi-structured data. SQL databases are not inherently faster, and the choice depends on the specific use case.

In Express.js, you can use the app.get('/route', callback) method to match routes with a specific pattern. The get method is used to handle HTTP GET requests and is often used to define routes with specific patterns. The other options do not represent the standard way to define route patterns in Express.js.

In modern ECMAScript standards, the for...of loop is the most suitable for iterating over an array and modifying each element. It provides a cleaner syntax and avoids issues with unexpected behavior that can occur with the for...in loop when working with arrays. The for loop and while loop are also options, but the for...of loop is more concise and specifically designed for array iteration.

In JavaScript, the import statement cannot be used inside conditionals (e.g., if statements or loops). It must be used at the top level of a module. This is because module imports are processed before the code is executed, and conditionals are not allowed to change the module structure dynamically.

In Jest, you can simulate user actions like clicks or keyboard inputs using jest.fn(). This allows you to create mock functions that can simulate user interactions and track their calls. The other options have different purposes; jest.spyOn() is used to spy on method calls, jest.mock() is used to mock modules, and jest.simulate() is not a valid Jest method for simulating user actions.

Cache bursting is a situation where multiple requests are made to a resource that is expensive to produce, causing a surge in load. It typically occurs when cached data expires or when the cache is invalidated.