go 45 lines · 7 steps

Parsing untyped JSON config in Go

How to safely walk a dynamic JSON blob and build a map of service URLs using type assertions.

Explained by highlit
Intermediate json parsing type assertions error handling defaults
1package config
2 
3import (
4 "encoding/json"
5 "fmt"
6)
7 
8func ExtractServiceEndpoints(raw []byte) (map[string]string, error) {
9 var doc map[string]interface{}
10 if err := json.Unmarshal(raw, &doc); err != nil {
11 return nil, fmt.Errorf("invalid config json: %w", err)
12 }
13 
14 services, ok := doc["services"].(map[string]interface{})
15 if !ok {
16 return nil, fmt.Errorf("missing or malformed 'services' object")
17 }
18 
19 endpoints := make(map[string]string, len(services))
20 for name, entry := range services {
21 cfg, ok := entry.(map[string]interface{})
22 if !ok {
23 continue
24 }
25 
26 host, _ := cfg["host"].(string)
27 if host == "" {
28 return nil, fmt.Errorf("service %q has no host", name)
29 }
30 
31 port := 80
32 if p, ok := cfg["port"].(float64); ok {
33 port = int(p)
34 }
35 
36 scheme := "http"
37 if tls, ok := cfg["tls"].(bool); ok && tls {
38 scheme = "https"
39 }
40 
41 endpoints[name] = fmt.Sprintf("%s://%s:%d", scheme, host, port)
42 }
43 
44 return endpoints, nil
45}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Unmarshaling into map[string]interface{} lets you handle JSON whose shape you don't know at compile time.
  2. 2Comma-ok type assertions turn risky casts into safe branches you can validate or default.
  3. 3JSON numbers always decode to float64 in Go, so numeric fields need an explicit conversion.

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