golang unit test failure expected zero, but got: "null\n"

Question content

First of all, I am trying to learn golang. I'm trying to unit test my handlers in golang using testify package. When running my test I get this error -> Expected nil, but got: "null\n" . I'm surprised it shows the method returning a null value. As far as I know, golangte has nil instead of null. what is the reason?

my handler test file

package entities

import (
    "github.com/google/uuid"
    "github.com/labstack/echo/v4"
    "github.com/stretchr/testify/assert"
    "net/http"
    "net/http/httptest"
    "strings"
    "testing"
)

type mock struct {
    create  func(user User) error
    getById func(id uuid.UUID) *User
}

var userJson = `{"id":"e918b0bc-76f3-4380-a1fb-a14ca5a08fc3","name":"Jon Snow","email":"[email protected]"}`

func (m *mock) Create(user User) error     { return m.create(user) }
func (m *mock) GetById(id uuid.UUID) *User { return m.getById(id) }

func TestCreate(t *testing.T) {
    t.Parallel()
    //Arrange
    e := echo.New()
    expectedStatusCode := http.StatusCreated
    m := &mock{create: func(user User) error { return nil }}
    req := httptest.NewRequest(http.MethodPost, "/", strings.NewReader(userJson))
    req.Header.Set(echo.HeaderContentType, echo.MIMEApplicationJSON)
    rec := httptest.NewRecorder()
    c := e.NewContext(req, rec)
    server := NewUserHandler(m)

    //Act
    server.Create(c)

    //Assert
    assert.Equal(t, expectedStatusCode, rec.Code)
    assert.Nil(t, rec.Body.String())
}


my handler

func (u *userHandler) Create(e echo.Context) error {

    var user User
    err := json.NewDecoder(e.Request().Body).Decode(&user)
    if err != nil {
        e.JSON(http.StatusUnprocessableEntity, err)
    }
    u.userService.Create(user)
    if err != nil {
        e.JSON(http.StatusBadRequest, err)
    }
    return e.JSON(http.StatusCreated, nil)
}

 my user service

 func (u *userService) Create(user User) (err error) {
    err = u.userRepo.Create(user)
    return
}

Correct answer

You are sending back JSON. When you marshal nil, it becomes "null".

In your tests you would do this

assert.Nil(t, rec.Body.String())

and rec.Body is JSON encoded nil (i.e. "null")

In fact, because rec.Body.String() returns string instead of *string, this test will never pass.

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