Example of unit-test on golang using testify

user.go

package example

type User struct {
	ID    string
	Email string
}

type SomeAPIClient interface {
	GetUser(usedId string) (*User, error)
}

func getUser(c SomeAPIClient, userID string) (*User, error) {
	return c.GetUser(userID)
}

user_test.go

package example

import (
	"errors"
	"testing"

	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/mock"
)

type FakeClient struct {
	mock.Mock
}

func (f FakeClient) GetUser(usedId string) (*User, error) {
	args := f.Called(usedId)
	if args.Get(0) == nil {
		return nil, args.Error(1)
	}
	return args.Get(0).(*User), args.Error(1)
}

func TestClient(t *testing.T) {
	assert := assert.New(t)
	testClient := new(FakeClient)

	testClient.On("GetUser", "error").Return(nil, errors.New("error"))
	u, err := getUser(testClient, "error")

	assert.Nil(u)
	assert.EqualError(err, "error")

	testUser := &User{ID: "user"}
	testClient.On("GetUser", "user").Return(testUser, nil)

	u, err = getUser(testClient, "user")

	assert.Equal(testUser, u)
	assert.Nil(err)

	testClient.AssertExpectations(t)
}