Questions
4 of 4
Explain how encoding/json marshaling works with struct tags and custom marshalers.
Difficulty: 5/10
struct tags, custom MarshalJSON, omitempty behavior
encoding/json uses struct tags to control JSON field names and behavior. Implement json.Marshaler and json.Unmarshaler interfaces for custom serialization logic.
Struct tags and custom marshalers
Common pitfalls
- 1
Unexported fields are silently ignored — always export fields you want serialized
- 2
interface{} numbers unmarshal as float64 by default — use UseNumber() for large integers
- 3
omitempty omits zero values: 0, false, "", nil — careful with intentional zero values
- 4
Anonymous embedded structs promote their fields to the parent — useful but can cause field collisions
- 5
json.RawMessage allows deferring or passing through raw JSON without parsing
Scenario Questions
0-2 years experience
- 1You need to serialize a struct to JSON but want the field `UserID` to appear as `user_id` in the output. How would you write the struct tag?
- 2If you add `omitempty` to a struct tag, what JSON will be produced when the field has its zero value?
- 3You have a type that implements `MarshalJSON`. What will json.Marshal do when encoding a value of that type?
2-5 years experience
- 1We added a custom `MarshalJSON` to a struct to format a timestamp, but after a recent change the JSON output is missing some fields. Walk me through how you'd debug this.
- 2When integrating with an external API, you need to omit empty slices but include empty maps. How would you configure struct tags or custom marshalers to achieve that?
- 3Explain the trade‑offs between using struct tags with `omitempty` versus writing a custom `MarshalJSON` for a large struct with many optional fields.
5-8 years experience
- 1Our service processes millions of JSON messages per second. Discuss the performance implications of using custom `MarshalJSON` implementations versus relying on the default encoder with tags.
- 2We need to evolve a public API's JSON schema without breaking existing clients. How would you use struct tags and custom marshalers to deprecate a field while still supporting older versions?
- 3Describe how you would design a reusable library that provides common marshal/unmarshal behavior (e.g., snake_case conversion, time formatting) across many services, considering maintainability and testing.
8+ years experience
- 1Our organization is migrating from a legacy JSON library to Go's `encoding/json`. What architectural considerations would you address regarding struct tags, custom marshalers, and backward compatibility across multiple microservices?
- 2How would you set up a cross‑team guideline for handling JSON field naming conventions and custom marshaling to ensure consistency and avoid subtle bugs at scale?
- 3If you had to support both JSON and protobuf representations for the same data structures, how would you reconcile struct tags and custom marshalers to minimize duplication?
Follow-up Questions
- What happens if a field has both a json tag and a MarshalJSON implementation?
- How does omitempty decide whether a value is zero for different types?
- Can you control the order of fields in the generated JSON?