When Does `json.Unmarshal` Return an Error in Go?

When Does JSON Unmarshal to Structure Return an Error in Go?

In Go, the json.Unmarshal function attempts to decode JSON-formatted bytes into a target data structure. While it successfully parses and populates fields with matching names in the source JSON, it does not raise an error when values do not correspond to the target's structure.

However, json.Unmarshal will encounter errors in the following situations:

Syntax Error:

If the JSON input is syntactically incorrect, json.Unmarshal will return an error. For instance, if a required quotation mark is missing, the decoder will fail to parse the JSON.

type A struct {
    Name string `json:"name"`
}
data := []byte(`{"name":what?}`)
err := json.Unmarshal(data, &a)
fmt.Println(err)  // prints character 'w' looking for beginning of value

JSON Value Not Representable by Target Type:

If the type of a JSON value cannot be converted to the type of the corresponding field in the target structure, json.Unmarshal will return an error. For example, if a JSON boolean is trying to be assigned to a string field:

data := []byte(`{"name":false}`)
type B struct {
  Name string `json:"name"`
}
var b B
err = json.Unmarshal(data, &b)
fmt.Println(err) // prints cannot unmarshal bool into Go value of type string

Other Implementation Details:

Apart from syntactic and type conversion errors, json.Unmarshal may also return errors in other uncommon situations. Refer to the Go documentation for a more exhaustive list.

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