Strings

len does not return the number of characters: Strings

Maps

Maps don't free that much memory as they shrink.
Shoutouts to: https://100go.co/28-maps-memory-leaks/

func main() {
    n := 1_000_000
    m := make(map[int][128]byte)
    printAlloc()

    for i := 0; i < n; i++ { // Adds 1 million elements
        m[i] = [128]byte{}
    }
    printAlloc()

    for i := 0; i < n; i++ { // Deletes 1 million elements
        delete(m, i)
    }

    runtime.GC() // Triggers a manual GC
    printAlloc()
    runtime.KeepAlive(m) // Keeps a reference to m so that the map isn’t collected
}

func printAlloc() {
    var m runtime.MemStats
    runtime.ReadMemStats(&m)
    fmt.Printf("%d MB\n", m.Alloc/(1024*1024))
}

Will print:

0 MB
382 MB
287 MB

TL:DR; Go Maps grow but never shrink. To avoid this, re-assign the map.

Passing references and copies

As in other programming languauges, passing to functions might use copies or references Passing references and values

Almost identical functions, but the latter one will alter the original slice:

func alterPeople(arr []string) {
	arr = append(arr, "John")
}

func alterPeopleInplace(arr []string) {
	arr[0] = "John"
}

func main() {
	people := []string{"Max", "Tom"}

	alterPeople(people)
	fmt.Println(people) // ["Max", "Tom"]

	alterPeopleInplace(people)
	fmt.Println(people) // ["John", "Tom"]
}