Answer

In Go, variables declared without an explicit initializer automatically receive what are called zero values, which differ for different types:

• int — 0
• bool — false
• string — ""
• pointers, interfaces, slices, maps, channels, functions — nil

This is convenient (no garbage in memory), but it can be dangerous: for certain types, such as slices and maps, working with a nil value can lead to panics or bugs.

Example:

var m map[string]int
m["key"] = 1     // panic: assignment to entry in nil map

var s []int
fmt.Println(s, s == nil) // [] true   (you can add elements, but you cannot index)

A tricky question

What is the difference between a nil slice and a nil map/channel, and what bugs does this lead to?

The correct answer: A slice with a nil value can be passed to functions, used in append and range — this is safe, but working with a nil map will lead to a panic when trying to write (but reading is allowed and will return a zero value).

Example:

var m map[string]int
fmt.Println(m["no_key"]) // 0 — safe
m["key"] = 1 // panic!

var s []int
s = append(s, 42) // okay

Examples of real errors due to ignorance of the subtleties of the topic

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Story

Description: In a large project, uninitialized maps were used for data aggregation. During the first write, the application consistently crashed with a panic. The problem was discovered in production when new statistics were introduced.

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Story

Description: The service serialized an uninitialized slice, sending it as JSON in the API response. Instead of [], clients received null — it was necessary to change the schema on the frontend to handle both cases.

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Story

Description: In the caching module, a nil map was incorrectly compared to an empty map, leading to incorrect determination of whether elements existed. This resulted in unnecessary database calls, reducing performance.