Thank you for using ff4go! Feel free to report any issue or improvement 🙏

Installation

go get github.com/dorianneto/ff4go

Configuration

Create an ff4go.json file at your project root. Each flag supports the following fields:

{
  "flags": [
    {
      "name": "new-ui",
      "enabled": true,
      "description": "Enables the redesigned UI",
      "rules": {
        "users": ["user1", "user2"],
        "environments": ["development", "staging"],
        "percentage": 25.0,
        "endAt": "2026-12-31T23:59:59Z"
      }
    }
  ]
}

Rules

Field	Type	Description
users	[]string	List of user identifiers that the flag is active for.
environments	[]string	List of environment names that the flag is active for.
percentage	float64	Percentage of users (0–100) to roll out to. When set, takes precedence over users/environments lists and uses a deterministic hash so the same user always gets the same result.
endAt	string	RFC 3339 timestamp after which the flag is treated as disabled, regardless of other rules. Omit to disable expiry.

users and environments are only evaluated when percentage is not set.

Initialization

From a file

Reads ff4go.json from the process working directory.

m, err := ff4go.NewManagerFromFile()
if err != nil {
    panic(err)
}

From bytes

Useful when you load the configuration from a remote source, an environment variable, or embed it with //go:embed.

data := []byte(`{"flags":[{"name":"new-ui","enabled":true}]}`)

m, err := ff4go.NewManagerFromBytes(data)
if err != nil {
    panic(err)
}

API

// IsEnabled returns the flag's top-level enabled value.
m.IsEnabled("new-ui")

// HasFlag reports whether a flag with the given name exists.
m.HasFlag("new-ui")

// IsEnabledForUser returns true when the flag is enabled and the user
// matches the users rule (or falls within the percentage rollout).
m.IsEnabledForUser("new-ui", "user1")

// IsEnabledForEnvironment returns true when the flag is enabled and the
// environment matches the environments rule (or falls within the percentage rollout).
m.IsEnabledForEnvironment("new-ui", "development")

// IsEnabledForUserAndEnvironment returns true only when both the user
// and environment checks pass simultaneously.
m.IsEnabledForUserAndEnvironment("new-ui", "user1", "development")

All targeting methods return false when the flag does not exist, is disabled, or has expired (endAt is in the past).

Examples

Basic flag check

package main

import (
    "fmt"

    "github.com/dorianneto/ff4go"
)

func main() {
    m, err := ff4go.NewManagerFromFile()
    if err != nil {
        panic(err)
    }

    fmt.Println(m.IsEnabled("new-ui"))                                        // true
    fmt.Println(m.IsEnabledForUser("new-ui", "user1"))                        // true
    fmt.Println(m.IsEnabledForEnvironment("new-ui", "staging"))               // false
    fmt.Println(m.IsEnabledForUserAndEnvironment("new-ui", "user1", "development")) // true
}

Loading from bytes (embedded config)

package main

import (
    _ "embed"
    "fmt"

    "github.com/dorianneto/ff4go"
)

//go:embed ff4go.json
var flagsConfig []byte

func main() {
    m, err := ff4go.NewManagerFromBytes(flagsConfig)
    if err != nil {
        panic(err)
    }

    fmt.Println(m.IsEnabled("new-ui")) // true
}

Percentage rollout

{
  "flags": [
    {
      "name": "new-checkout",
      "enabled": true,
      "rules": { "percentage": 10.0 }
    }
  ]
}

// The result is deterministic per (flag name, user id) pair.
fmt.Println(m.IsEnabledForUser("new-checkout", "user42")) // stable true/false

Scheduled flag expiry

{
  "flags": [
    {
      "name": "beta-banner",
      "enabled": true,
      "rules": {
        "users": ["tester1"],
        "endAt": "2026-06-01T00:00:00Z"
      }
    }
  ]
}

After 2026-06-01T00:00:00Z all targeting methods return false for beta-banner.