substack-concurrency-4.go

var wg sync.WaitGroup
sayHello := func() {
    defer wg.Done() // <- before we exit the goroutine, we indicate 
                    // to the WaitGroup that we have exited
    fmt.Println("hello")
}
wg.add(1) // <- one goroutine is starting
go sayHello() 
wg.Wait() // <---- join point