How to Distinguish Between JSON Null and Missing Fields in Go?

JSON Null vs. Missing Fields in Go: A Comparison

When working with JSON data in Go, it's crucial to distinguish between fields that are explicitly set to null and fields that are simply missing from the JSON input. This distinction becomes necessary when the JSON data represents a Golang struct.

Both null and missing fields result in nil values in the corresponding Go struct fields. However, discerning between the two scenarios can be critical for determining whether a field was intentionally set to null or was not included in the JSON data.

To address this challenge, two approaches are available:

Using Optional Structs with Generics (Go 1.18 )

Go 1.18 introduces generics, enabling the creation of a generic Optional struct to handle this situation. The Optional struct features a Defined boolean field indicating the presence of a value in the JSON input and a Value pointer to the actual value.

type Optional[T any] struct {
    Defined bool
    Value   *T
}

type Payload struct {
    Field1 Optional[string] `json:"field1"`
    Field2 Optional[bool]   `json:"field2"`
    Field3 Optional[int32]  `json:"field3"`
}

Using Custom Optional Structs (Pre-Generics)

Prior to generics, a custom Optional* struct can be created for each data type that needs to be handled as optional.

type OptionalString struct {
    Defined bool
    Value   *string
}

type Payload struct {
    SomeField1 string         `json:"somefield1"`
    SomeField2 OptionalString `json:"somefield2"`
}

By using these techniques, developers can easily discern whether a JSON field was explicitly set to null or was absent from the input data, providing greater flexibility and control over data validation and processing.

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