What happens when calling the append method and why does it return a new slice?

In Go, the append method is used to add elements to a slice. When calling append, the following happens:

1. If the slice has enough capacity, the new element is simply added to the existing array, and a slice pointing to the updated part of the array is returned.
2. If the capacity is insufficient, a new array with increased capacity is created, the old elements are copied into it, and then the new element is added.

Therefore, append returns a new slice because:

• In case of capacity expansion, it returns a slice pointing to a new array.
• When capacity is sufficient, it returns a slice pointing to the same array but with an increased length.

It is important to always assign the result of append back to a variable to avoid losing the reference to the updated slice:

s := []int{1, 2, 3}
s = append(s, 4)  // s now points to a slice with elements [1 2 3 4]

Thus, append returns a new slice because the underlying array may change internally, and the reference needs to be updated.