Answer.

In Go, the map structure is always passed by value, but this value contains a pointer to the internal structures of the map itself (under the hood — a hash table). Therefore, if you pass a map to a function and modify its contents inside the function (add/remove elements), the changes will be visible outside. Slices behave similarly — the slice structure is copied, but not the actual data array: the slice contains a pointer to the array, its length, and capacity. If you modify values by index inside the function, the original array will also change. However, if you assign a new slice from within the function (for example, reslicing), the original variable will not change.

func updateMap(m map[string]int) {
    m["key"] = 42 // Changes are visible outside!
}
func updateSlice(s []int) {
    s[0] = 99 // The original array is modified
}

Trick Question.

Why do changes to maps and slices inside a function affect the "original" when passed to the function?

Answer: Because only the pointer structure is copied, while the actual data remains shared — any changes affect the same memory block.

Examples of real errors due to misunderstanding of this topic.

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Story

In a fintech project, a map with configuration settings was passed to different services, assuming that local changes would not impact the shared map. As a result, one service altered values, leading to bugs in other modules that received unexpectedly changed configurations.

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Story

In an analytics microservice, a slice was passed, expecting to get independent copies. But the function inside changed values by index, leading to distorted data in the report, as the original array was unexpectedly modified.

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Story

In game servers, a map was used to store user sessions and was concurrently passed to several goroutines. It was assumed that each goroutine was working with its copy, but in reality, a data race occurred and sessions were partially lost.