Go Defer - howtogo.dev

defer fmt.Println("done") doesn't run immediately. It runs after greet's other statements finish, right before the function actually returns.

func greet() {
    defer fmt.Println("done")
    fmt.Println("hello")
}

Multiple defers stack up and run in last-in, first-out order. The last defer registered is the first one executed.

defer fmt.Println("first")
defer fmt.Println("second")
defer fmt.Println("third")

A deferred call's arguments are evaluated immediately, at the point of the defer statement, not when it eventually runs. Each loop iteration captures that iteration's value of i.

for i := 0; i < 3; i++ {
    defer fmt.Println(i)
}

A deferred closure can modify a named return value after the return statement has set it, since the return value isn't finalized until every deferred call has run.

func doSomething() (result int) {
    defer func() {
        result *= 2
    }()
    return 5
}

doSomething returns 10, not 5: the deferred closure doubles result after return 5 assigns it but before the function actually exits.

Terminal

$ go run defer.go
hello
done
2
1
0
12