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

When Does json.Unmarshal Return an Error in Go?

In Go, the json.Unmarshal function parses a JSON string into a struct, but it doesn't always return an error. However, it does return errors in certain situations.

When It Returns No Error

The JSON decoder typically doesn't report an error when values in the source don't match those in the target. For instance, if the source contains a field named "status" but the target doesn't, no error is raised.

Error Cases

json.Unmarshal does return errors in the following scenarios:

• Syntax errors: If the JSON string is malformed, such as missing quotes or punctuation.
• JSON value not representable by target type: If a JSON value can't be converted to the corresponding field type in the target struct. For example, trying to unmarshal a boolean value into a string field.

Example of Error Cases

type A struct {
    Name string `json:"name"`
}

var jsonString string = `{ "status": "false" }'`
var a A
error := json.Unmarshal([]byte(jsonString), &a)

fmt.Println(err) // prints cannot unmarshal bool into Go value of type string

In this example, an error is thrown because the source JSON has a "status" field which is of type boolean, but the target struct doesn't have a corresponding boolean field.

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