Fiber Binders

Binder is a new request/response binding feature for Fiber. Against the old Fiber parsers, it supports custom binder registration, struct validation, map[string]string, map[string][]string, and more. It's introduced in Fiber v3 and a replacement of:

• BodyParser
• ParamsParser
• GetReqHeaders
• GetRespHeaders
• AllParams
• QueryParser
• ReqHeaderParser

Default Binders

• Form
• Query
• URI
• Header
• Response Header
• Cookie
• JSON
• XML

Guides

Binding into the Struct

Fiber supports binding into the struct with gorilla/schema. Here's an example:

// Field names should start with an uppercase letter
type Person struct {
    Name string `json:"name" xml:"name" form:"name"`
    Pass string `json:"pass" xml:"pass" form:"pass"`
}

app.Post("/", func(c fiber.Ctx) error {
    p := new(Person)

    if err := c.Bind().Body(p); err != nil {
        return err
    }

    log.Println(p.Name) // john
    log.Println(p.Pass) // doe

    // ...
})

// Run tests with the following curl commands:

// curl -X POST -H "Content-Type: application/json" --data "{\"name\":\"john\",\"pass\":\"doe\"}" localhost:3000

// curl -X POST -H "Content-Type: application/xml" --data "<login><name>john</name><pass>doe</pass></login>" localhost:3000

// curl -X POST -H "Content-Type: application/x-www-form-urlencoded" --data "name=john&pass=doe" localhost:3000

// curl -X POST -F name=john -F pass=doe http://localhost:3000

// curl -X POST "http://localhost:3000/?name=john&pass=doe"

Binding into the Map

Fiber supports binding into the map[string]string or map[string][]string. Here's an example:

app.Get("/", func(c fiber.Ctx) error {
    p := make(map[string][]string)

    if err := c.Bind().Query(p); err != nil {
        return err
    }

    log.Println(p["name"])     // john
    log.Println(p["pass"])     // doe
    log.Println(p["products"]) // [shoe, hat]

    // ...
})
// Run tests with the following curl command:

// curl "http://localhost:3000/?name=john&pass=doe&products=shoe,hat"

Behaviors of Should/Must

Normally, Fiber returns binder error directly. However; if you want to handle it automatically, you can prefer Must().

If there's an error it'll return error and 400 as HTTP status. Here's an example for it:

// Field names should start with an uppercase letter
type Person struct {
    Name string `json:"name,required"`
    Pass string `json:"pass"`
}

app.Get("/", func(c fiber.Ctx) error {
    p := new(Person)

    if err := c.Bind().Must().JSON(p); err != nil {
        return err 
        // Status code: 400 
        // Response: Bad request: name is empty
    }

    // ...
})

// Run tests with the following curl command:

// curl -X GET -H "Content-Type: application/json" --data "{\"pass\":\"doe\"}" localhost:3000

Defining Custom Binder

We didn't add much binder to make Fiber codebase minimal. If you want to use your own binders, it's easy to register and use them. Here's an example for TOML binder.

type Person struct {
    Name string `toml:"name"`
    Pass string `toml:"pass"`
}

type tomlBinding struct{}

func (b *tomlBinding) Name() string {
    return "toml"
}

func (b *tomlBinding) MIMETypes() []string {
    return []string{"application/toml"}
}

func (b *tomlBinding) Parse(c fiber.Ctx, out any) error {
    return toml.Unmarshal(c.Body(), out)
}

func main() {
    app := fiber.New()
    app.RegisterCustomBinder(&tomlBinding{})

    app.Get("/", func(c fiber.Ctx) error {
        out := new(Person)
        if err := c.Bind().Body(out); err != nil {
            return err
        }

        // or you can use like:
        // if err := c.Bind().Custom("toml", out); err != nil {
        //      return err
        // }

        return c.SendString(out.Pass) // test
    })

    app.Listen(":3000")
}

// curl -X GET -H "Content-Type: application/toml" --data "name = 'bar'
// pass = 'test'" localhost:3000

Defining Custom Validator

All Fiber binders supporting struct validation if you defined validator inside of the config. You can create own validator, or use go-playground/validator, go-ozzo/ozzo-validation... Here's an example of simple custom validator:

type Query struct {
    Name string `query:"name"`
}

type structValidator struct{}

func (v *structValidator) Engine() any {
    return ""
}

func (v *structValidator) ValidateStruct(out any) error {
    out = reflect.ValueOf(out).Elem().Interface()
    sq := out.(Query)

    if sq.Name != "john" {
        return errors.New("you should have entered right name!")
    }

    return nil
}

func main() {
    app := fiber.New(fiber.Config{StructValidator: &structValidator{}})

    app.Get("/", func(c fiber.Ctx) error {
        out := new(Query)
        if err := c.Bind().Query(out); err != nil {
            return err // you should have entered right name!
        }
        return c.SendString(out.Name)
    })

    app.Listen(":3000")
}

// Run tests with the following curl command:

// curl "http://localhost:3000/?name=efe"