Quick Make Unit Test for your Spaghetty Code

var.go

package unittest

import (
	"context"
	"errors"
	"log"
	"time"

	"github.com/tokopedia/kunyit/src/conf"
	"github.com/tokopedia/sqlt"
)

type User struct {
	ID       int64
	Username string
	Status   int
}

var GetUser = func(id int64) (User, error) {
	var user User
	ctx, cancel := context.WithTimeout(context.Background(), time.Duration(conf.QueryTimeout)*time.Second)
	defer cancel()

	oauthClient := conf.DB.OauthClient

	resp, err := oauthClient.GetBasicUserInfoClientCredentialsWithContext(ctx, id)
	if err != nil {
		log.Println(err)
		return user, err
	}

	if resp.Error != "" {
		err = errors.New(resp.Error)
		return user, err
	}

	return user, nil
}

var InsertTalk = func(uid int64, message string) error {
	var err error

	db, err := sqlt.Open("postgres", "http://127.0.0.1")
	if err != nil {
		return err
	}

	if err := db.Ping(); err != nil {
		return err
	}

	query := `INSERT INTO ws_talk (id, message) VALUE ($1, $2)`
	_, err = db.ExecContext(context.TODO(), query, uid, message)
	if err != nil {
		return err
	}

	return err
}

func CreateTalk(uid int64, message string) error {
	var err error
	var u User

	u, err = GetUser(uid)
	if err != nil {
		return err
	}

	err = InsertTalk(u.ID, message)
	if err != nil {
		return err
	}

	return err
}

var_test.go

package unittest

import (
	"database/sql"
	"fmt"
	"testing"
)

func TestCreateTalk(t *testing.T) {
	type args struct {
		uid     int64
		message string

		errGetUser    error
		errInsertTalk error
	}
	tests := []struct {
		name    string
		args    args
		wantErr bool
	}{
		{
			name: "test create talk success",
			args: args{
				uid:     100,
				message: "hello world",
			},
			wantErr: false,
		},
		{
			name: "test create talk - failed to insert",
			args: args{
				uid:           100,
				message:       "hellow world",
				errInsertTalk: sql.ErrConnDone,
			},
			wantErr: true,
		},
		{
			name: "test create talk - failed to get user",
			args: args{
				uid:           100,
				message:       "hellow world",
				errInsertTalk: sql.ErrConnDone,
				errGetUser:    fmt.Errorf("failed to retrieve token, empty access token/expires in"),
			},
			wantErr: true,
		},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {

			GetUser = func(uid int64) (User, error) {
				return User{ID: uid}, tt.args.errGetUser
			}

			InsertTalk = func(uid int64, message string) error {
				return tt.args.errInsertTalk
			}

			if err := CreateTalk(tt.args.uid, tt.args.message); (err != nil) != tt.wantErr {
				t.Errorf("CreateTalk() error = %v, wantErr %v", err, tt.wantErr)
			}
		})
	}
}

Instead of

func GetUser(id int64) (User, error)

We use

var GetUser = func(id int64) (User, error) {

And mock it in test

GetUser = func(uid int64) (User, error) {
    return User{ID: uid}, tt.args.errGetUser
}