How to Test Private (Unexported) Functions in Go

You test private functions in Go by defining a test file in the same package with the _test suffix, which grants access to unexported identifiers.

// mypackage/mypackage_test.go
package mypackage

import "testing"

func TestPrivateFunction(t *testing.T) {
    result := privateHelper(10)
    if result != 20 {
        t.Errorf("got %d, want 20", result)
    }
}

This approach works because the test file belongs to the same package (mypackage), allowing it to see and call functions like privateHelper that start with a lowercase letter.

In Go, you cannot test private functions from outside their package because they are hidden by design. To test them, you create a test file inside the same folder and declare it as part of the same package. This is like having a backstage pass that lets you see the rehearsal (private functions) before the show (public API) opens to the public.