The Ultimate Guide to JSON in Go

As a language designed for the web, Go provides extensive support for working with JSON data. JSON (JavaScript Object Notation) is an incredibly popular data exchange format whose syntax resembles simple JavaScript objects. It’s one of the most common ways for applications to communicate on the modern web.

Encoding and decoding with struct tags 🔗

Go takes a unique approach for working with JSON data. The best way to think about JSON data in Go is as an encoded struct. When you encode and decode a struct to JSON, the key of the JSON object will be the name of the struct field unless you give the field an explicit JSON tag.

type User struct {
    FirstName string `json:"first_name"` // key will be "first_name"
    BirthYear int `json:"birth_year"` // key will be "birth_year"
    Email string // key will be "Email"
}

Example marshal JSON from struct (encode) 🔗

The encoding/json package exposes a json.Marshal function that allows us to generate the JSON encoding of any value, assuming that type has an encoder implemented. The good news is, all the default types in Go have an encoder created out-of-the-box, and you’ll usually be working with structs containing default-type fields.

func Marshal(v interface{}) ([]byte, error)

As you can see, Marshal() takes a value as input, and returns the encoded JSON as a slice of bytes on success, or an error if something went wrong.

dat, _ := json.Marshal(`User{
    FirstName: "Lane",
    BirthYear: 1990,
    Email:     "example@gmail.com",
}`)
fmt.Println(string(dat))

// prints:
// {"first_name":"Lane","birth_year":1990,"Email":"example@gmail.com"}

Example unmarshal JSON to struct (decode) 🔗

func Unmarshal(data []byte, v interface{}) error

Similarly, the json.Unmarshal() function takes some encoded JSON data and a pointer to a value where the encoded JSON should be written, and returns an error if something goes wrong.

dat := []byte(`{
    "first_name":"Lane",
    "birth_year":1990,
    "Email":"example@gmail.com"
}`)
user := User{}
err := json.Unmarshal(dat, &user)
if err != nil {
    fmt.Println(err)
}
fmt.Println(user)
// prints:
// {Lane 1990 example@gmail.com}

Example - Go JSON HTTP server 🔗

Building a JSON API in Go is simple, you don’t even need a framework to get access to convenient high-level HTTP support. I typically start by writing two little helper functions, respondWithJSON and respondWithError.

func respondWithJSON(w http.ResponseWriter, code int, payload interface{}) error {
    response, err := json.Marshal(payload)
    if err != nil {
        return err
    }
    w.Header().Set("Content-Type", "application/json")
    w.Header().Set("Access-Control-Allow-Origin", "*")
    w.WriteHeader(code)
    w.Write(response)
    return nil
}

respondWithJSON makes it easy to send a JSON response by simply providing the handler’s ResponseWriter, an HTTP status code, and a payload to be marshaled (typically a struct).

func respondWithError(w http.ResponseWriter, code int, msg string) error {
    return respondWithJSON(w, code, map[string]string{"error": msg})
}

The respondWithError function wraps the respondWithJSON function and always sends an error message. Now let’s take a look at how to build a full HTTP handler.

func handler(w http.ResponseWriter, r *http.Request) {
    defer r.Body.Close()
    type requestBody struct {
        Email    string `json:"email"`
        Password string `json:"password"`
    }
    type responseBody struct {
        Token string `json:"token"`
    }

    dat, err := io.ReadAll(r.Body)
    if err != nil {
        respondWithError(w, 500, "couldn't read request")
        return
    }
    params := requestBody{}
    err = json.Unmarshal(dat, &params)
    if err != nil {
        respondWithError(w, 500, "couldn't unmarshal parameters")
        return
    }

    // do stuff with username and password

    respondWithJSON(w, 200, responseBody{
        Token: "example-auth-token",
    })
}

Since the json.Marshal and json.Unmarshal function work on the []byte type, it’s really easy to send those bytes over the wire or write them to disk.

Example - Reading and writing JSON files 🔗

I use JSON files to store configuration from time to time. Go makes it easy to read and write JSON files.

Write JSON to a file in Go 🔗

type car struct {
    Speed int    `json:"speed"`
    Make  string `json:"make"`
}
c := car{
    Speed: 10,
    Make:  "Tesla",
}
dat, err := json.Marshal(c)
if err != nil {
    return err
}
err = io.WriteFile("/tmp/file.json", dat, 0644)
if err != nil {
    return err
}

Read JSON from a file in Go

type car struct {
    Speed int    `json:"speed"`
    Make  string `json:"make"`
}
dat, err := io.ReadFile("/tmp/file.json")
if err != nil {
    return err
}
c := car{}
err = json.Unmarshal(dat, &c)
if err != nil {
    return err
}

Tag Options - Omitempty 🔗

When marshaling data you can leave out a key completely if the key’s value contains a zero value using the omitempty tag.

type User struct {
  FirstName string `json:"first_name,omitempty"`
  BirthYear int `json:"birth_year"`
}

// if FirstName = "" and BirthYear = 0
// marshaled JSON will be:
// {"birth_year":0}

// if FirstName = "lane" and BirthYear = 0
// marshaled JSON will be:
// {"first_name":"lane","birth_year":0}

Tag Options - Ignore field 🔗

As mentioned above, non-exported (lowercase) fields are ignored by the marshaler. If you want to ignore additional fields you can use the - tag.

type User struct {
    // FirstName will never be encoded
    FirstName string `json:"-"`
    BirthYear int `json:"birth_year"`
 }

Default encoding types 🔗

JSON and Go types don’t match up 1-to-1. Below is a table that describes the type relationships when encoding and decoding.

You will notice that the float32 and int types are missing. Don’t worry, you can certainly encode and decode numbers into these types, they just don’t have an explicit type in the JSON specification. For example, if you encode an integer in JSON, it’s guaranteed not to have a decimal point. However, if someone converts that JSON value to a floating-point number before you decode it, you’ll get a runtime error.

It’s rare to encounter an error when marshaling JSON data, but unmarshalling JSON can often cause errors. Here are some things to watch out for:

• Any type conflicts will result in an error. For example, you can’t unmarshal a string into a int, even if the string value is a stringified number: "speed": "42"
• A floating-point number can’t be decoded into an integer
• A null value can’t be decoded into a value that doesn’t have a nil option. For example, if you have a number field that can be null, you should unmarshal into a *int
• A time.Time can only decode an RFC 3339 string - other kinds of timestamps will fail

Custom JSON marshaling 🔗

While most types have a default way to encode and decode JSON data, you may want custom behavior from time to time. Luckily, the json.Marshal and json.Unmarshal respect the json.Marshaler and json.Unmarshaler interfaces. To customize your behavior you just need to overwrite their methods MarshalJSON and UnmarshalJSON respectively.

type Marshaler interface {
    MarshalJSON() ([]byte, error)
}

type Unmarshaler interface {
    UnmarshalJSON([]byte) error
}

One of the most common scenarios for me is want to encode and decode timestamps in a different format, usually due to interoperability with another language like JavaScript.

type Group struct {
    ID        string        `json:"id"`
    CreatedAt unixTimestamp `json:"created_at"`
}

type unixTimestamp time.Time

func (ut unixTimestamp) MarshalJSON() ([]byte, error) {
    s := strconv.Itoa(int(time.Time(ut).Unix()))
    return []byte(s), nil
}

func (ut *unixTimestamp) UnmarshalJSON(dat []byte) error {
    unix, err := strconv.Atoi(string(dat))
    if err != nil {
        return err
    }
    *ut = unixTimestamp(time.Unix(int64(unix), 0))
    return nil
}

func main() {
    g := Group{
        ID:        "my-id",
        CreatedAt: unixTimestamp(time.Unix(1619544689, 0)),
    }
    dat, _ := json.Marshal(g)
    fmt.Println(string(dat))
    // prints
    // {"id":"my-id","created_at":1619544689}

    newG := Group{}
    json.Unmarshal(dat, &newG)
    fmt.Println(newG)
    // prints
    // {my-id {0 63755141489 0x1694c0}}
}

Arbitrary JSON with map[string]interface 🔗

It’s unfortunate when this is the case, but sometimes we have to work with arbitrary JSON data. For example, you need to decode some JSON data, but you aren’t sure what the key structure or shape of the data is.

The best way to handle this case is to unmarshal the data into a map[string]interface{}

dat := []byte(`{
    "first_name": "lane",
    "age": 30
}`)
m := map[string]interface{}{}
json.Unmarshal(dat, &m)
for k, v := range m {
    fmt.Printf("key: %v, value: %v\n", k, v)
}

// prints
// key: first_name, value: lane
// key: age, value: 30

I want to point out that map[string]interface{} should only be used when you absolutely have to. If you have a priori knowledge of the shape of the data, please use a struct or some other concrete type. Avoid the dynamic typing provided by interfaces when working with JSON, if you want, you can always use anonymous structs for one-off usage.

Streaming JSON encodings 🔗

Sometimes you don’t have the luxury of reading all the JSON data to or from a []byte. If you need to be able to parse data as it’s streamed in or out of your program the encoding/json package provides Decoder and Encoder types.

func NewDecoder(r io.Reader) *Decoder
func NewEncoder(w io.Writer) *Encoder

Take a look at the following example. It decodes data from standard in, adds a new key "id" with a value of "gopher-man" and writes the result to standard out.

dec := json.NewDecoder(os.Stdin)
enc := json.NewEncoder(os.Stdout)
for {
    v := map[string]interface{}{}
    if err := dec.Decode(&v); err != nil {
        log.Fatal(err)
    }
    v["id"] = "gopher-man"
    if err := enc.Encode(&v); err != nil {
        log.Fatal(err)
    }
}

Pretty printing JSON 🔗

By default, the json.Marshal function compresses all the whitespace in the encoded data for efficiency. If you need to print out your JSON data so that it’s more easily readable you can pretty-print it using the json.MarshalIndent function.

func MarshalIndent(v interface{}, prefix, indent string) ([]byte, error)

You can customize how you want your pretty JSON to be formatted, but if you just want it to have proper tabs and newlines you can do the following.

type user struct {
    Name string
    Age  int
}

json, err := json.MarshalIndent(user{Name: "lane", Age: 30}, "", "  ")
if err != nil {
    return err
}

fmt.Println(string(json))
// prints
// {
//     "Name": "lane",
//     "Age": 30
// }

Faster JSON encoding and decoding 🔗

Sometimes performance is key, and when you need to squeeze every bit of power out of your CPU cycles you may notice that the standard library’s JSON marshaling and unmarshalling isn’t as performant as you might like. There’s a fairly popular library out there called ffjson that can speed up your marshaling performance by ~2-3x.

ffjson generates static MarshalJSON and UnmarshalJSON functions for structures in Go. The generated functions reduce the reliance upon runtime reflection to do serialization and are generally 2 to 3 times faster. In cases where ffjson doesn’t understand a Type involved, it falls back to encoding/json, meaning it is a safe drop in replacement. By using ffjson your JSON serialization just gets faster with no additional code changes.

ffjson Readme.md

If you’re thinking about using this library let me give you my quick two cents: I’ve never actually needed to speed up my JSON performance, it’s never been a bottleneck in my apps. If you desperately need to increase performance I think this is a great tool to look into, but don’t add needless dependencies to your code when the benefit they offer isn’t truly necessary.