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In Go’s traditional encoding/json API, a plain field or pointer does not reliably tell you whether a JSON member was omitted or explicitly set to null. To preserve all three states—missing, null, and a concrete value—decode the containing object into map[string]json.RawMessage, check whether the key exists, and then inspect or decode its value.
What Go’s usual struct decoding preserves
When decoding into a fresh struct with the traditional encoding/json package (often called v1), a member’s absence is not recorded as separate metadata. An omitted scalar field remains at its Go zero value. For scalar kinds, an explicit JSON null is ignored and also leaves the value unchanged. Those inputs therefore cannot be distinguished from the resulting scalar alone.
A pointer field preserves a useful but narrower distinction: whether a non-null value was decoded. In the common v1 pattern, both an omitted pointer field and a field explicitly set to null result in nil. The Go project’s JSON tutorial puts the absent-field behavior this way: “If there were a Bar field in the JSON object, Unmarshal would allocate a new Bar and populate it. If not, Bar would be left as a nil pointer.”
Use a pointer when nil versus a decoded non-null value is all your application needs. Do not treat it as a three-state presence detector.
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Choose a representation based on the states you need
| Requirement | Representation | What it preserves |
|---|---|---|
| Know whether a non-null value was decoded | *T field |
Nil versus a decoded non-null value; in the common v1 case, missing and explicit null both become nil. |
| Distinguish missing, null, and a concrete value | map[string]json.RawMessage plus key lookup and value decoding |
Member presence and the raw JSON payload, so the three states can be handled separately. |
| Keep a typed struct API and retain presence | Custom wrapper with UnmarshalJSON |
All three states if the wrapper explicitly records presence and whether the value is null. |
| Inspect an open-ended object before choosing fields | map[string]json.RawMessage or a generic JSON map |
Object-member presence; validate and decode selected values separately. |
Use RawMessage to detect all three states
Decode the object into a map of raw values. The map lookup’s second result tells you whether the key was present; the raw bytes tell you whether its value was JSON null. Decode any other value into the target type so malformed or incompatible input returns an error.
package example
import (
"bytes"
"encoding/json"
)
func readName(data []byte) error {
var fields map[string]json.RawMessage
if err := json.Unmarshal(data, &fields); err != nil {
return err
}
raw, present := fields["name"]
switch {
case !present:
// The member was missing.
case bytes.Equal(bytes.TrimSpace(raw), []byte("null")):
// The member was present with explicit JSON null.
default:
// The member was present with a non-null JSON value.
var name string
if err := json.Unmarshal(raw, &name); err != nil {
return err
}
_ = name // Use the decoded value.
}
return nil
}json.RawMessage is a byte slice, so comparing its whitespace-trimmed contents with null identifies the explicit null token. The example decodes the non-null branch as a string; use the actual field type in your code, and return its decoding error to the caller rather than silently accepting the wrong JSON type.
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The example assumes the top-level input should be an object. Decide separately what your endpoint should do if the whole input is JSON null or is a different JSON type, then validate that contract. With many fields or nested structures, repeated map checks can become cumbersome; a typed wrapper that records presence, null, and value—or a two-pass decode—may make the application API clearer.
Apply the distinction deliberately in PATCH-style requests
Go cannot infer what an omitted or null request field means for your application. A common PATCH contract is: missing means leave the stored value unchanged, null means clear it, and a concrete value means replace it. Define that contract for the endpoint, then use a representation that preserves the input state long enough for the handler to apply it.
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- Missing: do not modify the existing value if that is the endpoint’s contract.
- Null: clear the value only if the endpoint defines null that way.
- Concrete value: validate and apply the decoded replacement.
Decode into a newly initialized destination when behavior must not depend on prior contents. Otherwise, a field left unchanged during decoding may retain a value that was already in the destination, making the result depend on its earlier state.
Do not use omitempty to detect incoming fields
omitempty is a marshaling option, not a marker that records whether an input member appeared. In v1 it controls omission when encoding according to Go empty-value rules; it has no special role in detecting presence during unmarshaling. The v1 package documentation describes its encoding behavior.
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Check which JSON API your Go code uses
The behavior above is specifically for the traditional encoding/json API (v1). Go documents encoding/json/v2 separately, and its semantics can differ, including for null handling and merging into preexisting values. The Go blog’s JSON v2 article notes that Go 1.27 introduces the v2 package. Check the documentation for the exact package and toolchain version in your project before applying v1 assumptions to v2.
The packages also define omitempty differently when marshaling: v1 uses Go empty-value rules, while v2 uses empty JSON values. Neither package’s option is an unmarshaling presence detector.
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