How to test the printNumber function without waiting a second?

To test the printNumber function, which internally calls time.Sleep(time.Second), without actually waiting for a second, you can use dependency injection for the sleep function. Instead of directly calling time.Sleep in the code, pass the sleep function as a parameter or use an interface.

Example:

package main

import (
    "fmt"
    "time"
)

type Sleeper func(d time.Duration)

func printNumber(sleep Sleeper, n int) {
    sleep(time.Second) // instead of directly calling time.Sleep
    fmt.Println(n)
}

// In the test, pass a stub:
func main() {
    fakeSleep := func(d time.Duration) {
        // do nothing, to avoid waiting
    }
    printNumber(fakeSleep, 42) // will immediately print 42
}

Thus, in tests, you can replace the real sleep with an empty function, which speeds up testing and eliminates delays.