Working as intended, you need to use the result of append as it may reallocate the underlying array if there is insufficient space for new elements.

See: https://pkg.go.dev/builtin#append

I disagree. arr[:2] does not have the capacity to store a new element. So by the documentation, it should allocate a new array and not change arr for some reason. However, if it is intended, I'll just keep in mind, that append may change other slices too.

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@Vector-Hector arr[:2] does have the capacity to store "e". Use the 3 argument form of a slice expression, a[low : high : max], if you intend on controlling capacity: https://go.dev/ref/spec#Slice_expressions.

https://play.golang.com/p/UNFoS592mq5 prints "3 vs 2":

func main() {
	c := []string{"a", "b", "c"}
	fmt.Println(cap(c[:2]), " vs ", cap(c[:2:2]))
}