Return an empty array instead of null with golang for json return with gin

Question

I have a struct:

type ProductConstructed struct {
    Name string `json:"Name"`
    BrandMedals []string `json:"BRAND_MEDALS"`
}

When I return my object with gin:

func  contructproduct(c *gin.Context) {
    var response ProductConstructed 
    response.Name = "toto"

    c.JSON(200, response)
}

func main() {
    var err error
    if err != nil {
        panic(err)
    }
    //gin.SetMode(gin.ReleaseMode)
    r := gin.Default()
    r.POST("/constructProductGo/v1/constructProduct", contructproduct)
    r.Run(":8200") // listen and serve on 0.0.0.0:8080
}

It returns:

null

instead of:

[]

How to return an empty array?

You are using struct for response instead of array of struct so current output is accurate one. It seems from your code that you want to return only one struct at a time. So instead of handling it via empty array, handle it on null. — Manish Champaneri
– Manish Champaneri, Commented May 20, 2019 at 6:54

Bussiere · Accepted Answer · 2019-05-18 17:00:35Z

69

So the solution was to initialize it with :

productConstructed.BrandMedals = make([]string, 0)

answered May 18, 2019 at 17:00

Bussiere

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3 Comments

Seaskyways Over a year ago

or productConstructed.BrandMedals = []string{} would do too

Bussiere Over a year ago

@Seaskyways your solution is more elegant and lisible thanks

Seaskyways Over a year ago

it is 100% equivalent, just different syntax

frost · Accepted Answer · 2021-08-06 20:52:43Z

27

Just to clarify why the solution above works:

slice1 is declared, but not defined. No variable has been assigned to it yet, it equals nil. When serialized, it will return null.

slice2 is declared and defined. It equals to an empty slice, not nil. When serialized, it will return [].

var slice1 []string  // nil slice value
slice2 := []string{} // non-nil but zero-length

json1, _ := json.Marshal(slice1)
json2, _ := json.Marshal(slice2)

fmt.Printf("%s\n", json1) // null
fmt.Printf("%s\n", json2) // []

fmt.Println(slice1 == nil) // true
fmt.Println(slice2 == nil) // false

Go playground: https://play.golang.org/p/m9YEQYpJLdj

More info: https://github.com/golang/go/wiki/CodeReviewComments#declaring-empty-slices

answered Aug 6, 2021 at 20:52

frost

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3 Comments

orpheus Over a year ago

This was a nice and simple answer. I did a if len(records) == 0 { return []record{} }

Andrey Nikishaev Over a year ago

That's incorrect behavior since go use default values for all vars that not a pointer. go.dev/play/p/lOl4w2FEHI0

Jerome Doucet Over a year ago

@AndreyNikishaev slice, like map and chan "are" (to make it simple) pointers in Go.

tim-montague · Accepted Answer · 2024-09-05 23:21:44Z

1

To return an empty list [] use:

response.BrandMedals = []string{}

answered Sep 5, 2024 at 23:21

tim-montague

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