EC
json.UnmarshalTypeError
GoERRORNotableJSON

cannot unmarshal … into Go value of type …

Quick Answer

Type-assert to *json.UnmarshalTypeError to log the Field name and fix the struct definition.

What this means

Returned when a JSON value cannot be stored in the target Go type, such as a JSON string into an int field. Field and Offset identify the problem location.

Why it happens
  1. 1JSON field type does not match the Go struct field type
  2. 2Numeric string received where a number was expected

Fix

Type-assert for details

Type-assert for details
var te *json.UnmarshalTypeError
if errors.As(err, &te) {
    log.Printf("wrong type for field %s", te.Field)
}

Why this works

The Field name guides developers to the struct or JSON property that needs correction.

Code examples
Detectgo
type T struct{ Age int }
var t T
err := json.Unmarshal([]byte(`{"Age":"old"}`), &t)
var te *json.UnmarshalTypeError
errors.As(err, &te)
Use json.Numbergo
dec := json.NewDecoder(r)
dec.UseNumber()
dec.Decode(&v)
Custom unmarshalergo
func (t *T) UnmarshalJSON(b []byte) error {
    // custom parsing logic
    return nil
}
Sources
Official documentation ↗

Go standard library

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