Go Structs from JSON: Tags and Types

Go's encoding/json package maps JSON to structs using struct tags. Getting the types and tags right is tedious for large API responses. Here's how the mapping works and how to automate it.

Basic Mapping

The json:"name" tag tells the JSON decoder which JSON key maps to which struct field. Without tags, Go uses the field name directly (case-insensitive matching).

JSON to Go Type Mapping

Nested Structs

JSON objects within objects become separate Go structs:

Handling Optional Fields

omitempty

omitempty omits the field from JSON output if it has a zero value:

Pointer types for nullable JSON

If a JSON field can be null, use a pointer:

Without a pointer, null and missing both become the zero value ("" for string), and you can't tell them apart.

Arrays and Slices

JSON arrays become Go slices:

Mixed-type arrays ([1, "two", true]) are uncommon in well-designed APIs but map to []interface{}.

Automated Generation

For large API responses, hand-writing structs is error-prone. Our JSON to Go converter generates complete struct definitions:

1. Paste your JSON response
2. Get properly tagged Go structs with correct types
3. Nested objects are extracted into separate named structs
4. Array element types are inferred from the data

The output is ready to paste into your Go project.

Common Patterns

Decoding API responses

Ignoring unknown fields

By default, encoding/json silently ignores JSON keys that don't match any struct field. To enforce strict decoding:

Renaming fields

Go convention is PascalCase, but JSON APIs use camelCase or snake_case. Tags handle the mapping:

Tips

1. Always add json tags. Without them, you depend on Go's case-insensitive matching, which is fragile.
2. Use pointers for optional fields that you need to distinguish from zero values.
3. Generate from real API responses, not docs. The actual response structure is what matters.
4. Test with json.Marshal to verify your struct produces the expected JSON output.