When dealing with a legacy codebase without existing tests, the best approach is to start by creating unit tests for individual functions and methods. This allows you to isolate and test specific pieces of code in isolation. Once you have a solid base of unit tests, you can gradually introduce integration tests and end-to-end tests as needed. Using test doubles like mocks and stubs can help isolate dependencies, and property-based testing can be beneficial, but it's usually not the initial step in creating tests for a legacy codebase.

The primary purpose of interfaces in Go is to enable code reusability and achieve polymorphism. They allow you to write code that can work with different types as long as they satisfy the interface contract. This promotes flexibility in your codebase, making it easier to swap implementations and extend functionality. Interfaces also facilitate testing and mocking, as you can create custom implementations that conform to the same interface. This promotes clean, modular, and maintainable code in Go.

Ensuring that a mock object behaves as expected involves verifying that the methods of the mock object are called with the correct arguments and return the expected values. This can be achieved by using assertions in the test code. Candidates should explain the importance of setting up expectations for method calls and return values and using assertions to validate that these expectations are met during testing. They may also mention the use of testing frameworks like Go's testing package or third-party libraries for mocking, such as "github.com/stretchr/testify/mock."

The built-in error type in Go is simply named error. In Go, errors are represented as values of the error interface type. This interface defines a single method called Error() string, which is used to convert an error value to a human-readable string. By returning an error value from functions, Go provides a simple and idiomatic way to handle and propagate errors throughout the code.

In Go, := is used for short declaration and assignment, which creates a new variable and infers its type from the assigned value. On the other hand, the var keyword is used for variable declaration, where you must explicitly specify the type. For example, x := 10 creates a new variable x with an inferred type of int, while var y int declares a variable y of type int.

In Go, you can make a copy of a slice by assigning the original slice to a new variable. However, it's essential to understand that this does not create a deep copy; both the original and the new variable will reference the same underlying array. Modifying elements in one will affect the other. To create a true copy, you can use the copy() function or create a new slice and append elements from the original slice.

In Go testing, the t.Skip function is used to indicate that a test should be skipped. This is particularly useful when certain conditions are not met, and you want to skip the execution of a specific test. It helps in handling scenarios where a test is not applicable or meaningful in certain contexts.

Mock objects in Go can be created to implement interfaces for testing purposes. When writing unit tests, it's often necessary to isolate the code being tested from external dependencies, such as databases or web services. Mock objects that implement the same interfaces as the real dependencies can be used to simulate their behavior, allowing for controlled and repeatable testing. This technique is common in unit testing to ensure that the code under test interacts correctly with its dependencies.

The http.ResponseWriter and http.Request parameters in a handler function serve essential roles. The http.ResponseWriter allows you to write the HTTP response back to the client's browser. You can use it to set headers, status codes, and send content to the client. The http.Request parameter represents the incoming HTTP request and provides access to request data such as URL parameters, headers, and form values. These two parameters together enable you to process incoming requests and generate appropriate responses, making them integral to building web applications in Go.

Utilizing Goroutines can significantly improve program performance when performing parallel tasks like web scraping. In such scenarios, multiple web requests can be made concurrently, reducing the overall time needed to fetch data. By creating a Goroutine for each request, the program can efficiently utilize available resources and complete tasks much faster than if it were done sequentially. Web scraping is a common use case where Goroutines shine.