json unmarshal "null" to empty slice variable(etc: []string{} ) doesn't have err. But when marshal the variable, the value will be "null" instead of "[]"

[Tron2016]

What version of Go are you using (go version)?

go version go1.10.2

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

linux/amd64

What did you do?

ls := []string{}
if err := json.Unmarshal([]byte("null"), &ls); err != nil {
	fmt.Println(err)
}
fmt.Println(ls)
if result, err := json.Marshal(ls); err != nil {
	fmt.Println(err)
} else {
	fmt.Println(string(result))
}

What did you expect to see?

output:
[]
[]

What did you see instead?

output
[]
null

[mvdan]

This is a common misconception in the encoding/json package, but it is very clearly documented:

Array and slice values encode as JSON arrays, except that []byte encodes as a base64-encoded string, and a nil slice encodes as the null JSON value.

If you want to marshal a slice as an empty JSON array, you need to make the slice be non-nil. This has been covered at length before: https://stackoverflow.com/questions/33183071/golang-serialize-deserialize-an-empty-array-not-as-null