REST stands for "Representational State Transfer." It is not a protocol but rather an architectural style that defines a set of constraints for creating scalable and maintainable web services. RESTful APIs use HTTP as a communication protocol and adhere to these constraints, making it easier to develop and consume APIs in a consistent and efficient manner. Understanding these constraints is essential for designing RESTful APIs effectively.

Custom errors provide a way to "encapsulate" more information about the error conditions. By encapsulating errors, you can wrap the original error with additional context and information, making it easier to handle and communicate the error's details to other parts of the application or to external systems. This helps improve error reporting, debugging, and overall system resilience.

A race condition is a critical concurrency issue where two or more goroutines access shared data without proper synchronization, leading to unpredictable and erroneous behavior. The -race flag in Go is a powerful tool for detecting race conditions. When you compile your Go program with the -race flag, the Go runtime will instrument your code to track all accesses to shared variables. If it detects a race condition during runtime, it will report it, helping you identify and fix the issue. The -race flag is a vital tool for ensuring the correctness of concurrent Go programs.

A map's keys must be of a type that is both Comparable and Hashable. This requirement ensures that keys can be compared for equality and that they can be efficiently stored and retrieved from the underlying data structure. Comparable keys are necessary for searching and indexing, while Hashable keys allow for efficient lookup in the map. Failing to use keys of a compatible type can lead to unexpected behavior in map-based data structures.

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.

Type assertions in Go are primarily used to convert an interface type to a concrete type when you know the underlying type. This is useful when you need to access fields or methods specific to the concrete type. For example, when dealing with an interface{} that holds different types, you can use type assertions to safely extract and work with the actual types contained within the interface.

Dependency injection is crucial for writing testable Go code. When you inject dependencies into a Go function or struct, you can replace those dependencies with mock objects during testing. This allows you to isolate the code under test and write focused unit tests. By injecting dependencies, you decouple components, making it easier to test individual units without relying on the behavior of other components. In this way, dependency injection promotes unit testing and ensures that the code is testable, maintainable, and reliable.

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.