What happens when you append element in Go slice

What happens when you append to a slice and it exceeds its capacity?

When you use append() on a slice and the number of elements goes beyond its current capacity, Go does the following under the hood:

1. Allocates a New Underlying Array

Go automatically creates a new, bigger array (usually 2x the old capacity, but not always—depends on the implementation).

2. Copies Existing Elements

All the elements from the old slice are copied into the new array.

3. Adds the New Element

The new element is added to the new array.

4. Returns a New Slice

A new slice pointing to the new array is returned. The original slice is unchanged (but still points to the old array).

Example:

1s := make([]int, 0, 2)
2s = append(s, 1, 2)
3fmt.Println(cap(s)) // 2
4
5s = append(s, 3) // Capacity exceeded
6fmt.Println(cap(s)) // Capacity 4
7

1s := make([]int, 0, 2)
2s = append(s, 1, 2)
3fmt.Println(cap(s)) // 2
4
5s = append(s, 3) // Capacity exceeded
6fmt.Println(cap(s)) // Capacity 4
7

After that last append(), Go allocates a new array with larger capacity and returns a new slice pointing to it.

So, when capacity is exceeded, Go handles everything for you automatically.