The go fmt command is used for formatting Go source code to adhere to the Go programming style guidelines. It focuses solely on code formatting, such as indentation, spacing, and line breaks. On the other hand, go vet is used for checking for suspicious constructs in your code, like potential bugs, dead code, or suspicious variable usage. It primarily checks for code correctness and doesn't address code formatting issues.

In Go, the len function returns the length (the number of elements) of a slice, while the cap function returns the capacity (the maximum number of elements the slice can hold without resizing). These functions are crucial when working with slices because they allow you to determine the current size of a slice and its capacity for future growth. Understanding how to use len and cap helps you manage and manipulate slices effectively in your Go programs.

The Marshal and Unmarshal functions in Go are part of the encoding/json package. These functions are used to encode Go data structures into JSON format and decode JSON data into Go data structures, respectively. The encoding/json package provides the necessary functions and types for working with JSON data in Go, making it an essential package for handling JSON encoding and decoding operations in the language.

In Go testing, the t.Fatal function is used to terminate the current test immediately and report it as a failure. This function is typically used when a critical condition is not met during the test execution, and you want to indicate that the test cannot proceed successfully. It helps in identifying and diagnosing issues more precisely by stopping the test as soon as a problem is encountered, rather than continuing to execute the subsequent test code.

When working with files in Go, you should check for errors by using conditional statements. The correct option is if err != nil { log.Fatal(err) }. This checks if the err variable (commonly used for error handling) is not nil and, if not, logs the error and exits the program using log.Fatal(). Proper error handling is essential when dealing with file operations in Go.

In Go, the defer statement schedules a function to be called after the surrounding function returns. This is often used for tasks like closing files, releasing resources, or ensuring cleanup operations happen even in the presence of errors. When defer is used, the function call is deferred until the end of the enclosing function's scope, ensuring it runs just before that function returns.

In this scenario, you would prefer to use a Struct in Go to store information about each book. A Struct allows you to define a custom data type with fields to represent attributes of a book (e.g., title, author, ISBN). It provides a way to encapsulate related data and behaviors into a single unit, making it ideal for representing individual books in the library's inventory. Using a Struct allows you to access book properties using dot notation, making your code more organized and readable.

The correct answer is uppercase. In Go, when you encode a data structure into JSON using the encoding/json package, the fields in the data structure need to be exported, which means they need to start with an uppercase letter. Exported fields are those that are accessible from outside the package, and they are the only fields that the encoding/json package can encode into JSON. This convention is important for proper JSON encoding and decoding in Go.

Shadowing in Go refers to declaring a variable with the same name in an inner scope, which temporarily hides a variable of the same name in an outer scope. While not inherently problematic, it can lead to confusion and potential bugs. When shadowing occurs, it can be challenging to determine which variable is being accessed or modified. It's essential to be aware of shadowing to write clean and maintainable Go code and avoid unexpected behavior caused by variable conflicts.

When a Go interface becomes large and has many methods, it's often beneficial to split it into smaller interfaces for easier mocking and testing. This practice aligns with the SOLID principles, particularly the "Interface Segregation Principle" (ISP), which recommends that you should have many client-specific interfaces rather than a single, large, general-purpose one. Smaller interfaces make it easier to create focused mocks and test specific interactions.