How to write unit test failure for json.NewDecoder.Decode? - json

Question

I have to write unit tests for a function and this function uses json.NewDecoder.Decode
var infos models.RegisterInfos // struct with json fields
err := json.NewDecoder(r.Body).Decode(&infos)
if err != nil {
// do something
}
How can I simulate an error in a unit test (using the testing package) for json.NewDecoder(r.Body).Decode(&infos) ? I tried looking in the NewDecoder and Decode source code but I couldn't find anything that can generate an error in just a few lines.

Answer 1

you could send a body like <invalid json> as example:
func main() {
body := "<invalid json>"
var infos RegisterInfos // struct with json fields
err := json.NewDecoder(strings.NewReader(body)).Decode(&infos)
if err != nil {
fmt.Println(err)
}
}
See https://go.dev/play/p/44E99D0eQou

Answer 2

Feed it an invalid input, or decode into an invalid output:
package main
import (
"encoding/json"
"fmt"
"strings"
"testing"
)
type Message struct {
Name string
}
func TestDecodeFail(t *testing.T) {
for _, tc := range []struct {
in string
desc string
out any
}{
{`{Name: "Bobby"}`, "key without quotes", &Message{}},
{`{"Name": "Foo"a}`, "extra character", &Message{}},
{`{"Name": "Foo"}`, "bad destination", &struct{ Name int64 }{}},
{`{"Name": "Foo` + "\u001a" + `"}`, "invalid character", &Message{}},
{`{"Name": "Foo"}`, "unmarshal to nil", (*Message)(nil)},
} {
err := decode(tc.in, tc.out)
if err != nil {
fmt.Printf("%s -> %s, %T\n", tc.desc, err.Error(), err)
}
}
}
func decode(in string, out any) error {
return json.NewDecoder(strings.NewReader(in)).Decode(out)
}
Outputs:
key without quotes -> invalid character 'N' looking for beginning of object key string, *json.SyntaxError
extra character -> invalid character 'a' after object key:value pair, *json.SyntaxError
bad destination -> json: cannot unmarshal string into Go struct field .Name of type int64, *json.UnmarshalTypeError
invalid character -> invalid character '\x1a' in string literal, *json.SyntaxError
unmarshal to nil -> json: Unmarshal(nil *main.Message), *json.InvalidUnmarshalError