Dynamic JSON Keys to Go map Types

Handling Dynamic JSON Keys with Go Maps

When a JSON object has keys that are not known at compile time, you cannot define a struct. Instead, use Go's map types.

Example JSON

{
  "metrics": {
    "cpu_usage": 72.5,
    "memory_usage": 85.3,
    "disk_io": 45.1
  },
  "labels": {
    "env": "production",
    "region": "us-east-1",
    "team": "backend"
  }
}

Generated Go Struct

type AutoGenerated struct {
    Metrics map[string]float64 `json:"metrics"`
    Labels  map[string]string  `json:"labels"`
}

When Values Have Mixed Types

If values are not uniform, use map[string]interface{} (or map[string]any in Go 1.18+):

{
  "settings": {
    "theme": "dark",
    "fontSize": 14,
    "autoSave": true
  }
}

type Config struct {
    Settings map[string]interface{} `json:"settings"`
}

Type Assertions

When using interface{} values, you need type assertions to access the underlying type:

if theme, ok := config.Settings["theme"].(string); ok {
    fmt.Println("Theme:", theme)
}

Iterating Over Maps

for key, value := range response.Metrics {
    fmt.Printf("%s: %.1f\n", key, value)
}

Struct vs Map Decision

Use a struct when the keys are known and fixed. Use a map when keys are dynamic, user-defined, or vary between responses. In practice, many APIs combine both — fixed fields as struct fields and dynamic fields as a map.