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

Question

So i have a struct :

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

When i return my object with gin and :

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 me :

null

instead of

[]

How to return an empty array ?

Regards

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, May 20 '19 at 06:54

score 56 · Accepted Answer · answered May 18 '19 at 17:00

56

So the solution was to initialize it with :

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

answered May 18 '19 at 17:00

Bussiere

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or `productConstructed.BrandMedals = []string{}` would do too – Seaskyways May 20 '19 at 08:25
@Seaskyways your solution is more elegant and lisible thanks – Bussiere May 20 '19 at 13:34
it is 100% equivalent, just different syntax – Seaskyways May 20 '19 at 15:29

score 20 · Answer 2 · answered Aug 06 '21 at 20:52

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

This was a nice and simple answer. I did a `if len(records) == 0 { return []record{} }` — orpheus, Mar 06 '22 at 13:49
That's incorrect behavior since go use default values for all vars that not a pointer. https://go.dev/play/p/lOl4w2FEHI0 — Andrey Nikishaev, May 12 '22 at 10:47

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

2 Answers2