doc_format.go

// Copyright 2026 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

//go:build (!goexperiment.jsonv2 || !go1.25) && goexperiment.jsonformat

// # Format Tag Option
//
// The `format` tag option is experimental,
// and not subject to the Go 1 compatibility promise.
// It only exists when building with the GOEXPERIMENT=jsonformat environment variable set.
//
// Some Go types support alternative JSON representations as specified below.
// The `format` tag option is a key-value pair specified as "format:value"
// where the value must be either a literal consisting of letters and numbers
// (e.g., "format:RFC3339") or a single-quoted string literal
// (e.g., "format:'2006-01-02'"). The interpretation of the format option
// is determined by the struct field type.
//
// Go types with alternative representations are as follows:
//
//   - A Go []byte or [N]byte is usually represented as a JSON string
//     containing the binary value encoded using RFC 4648.
//     If the format is "base64" or unspecified, then this uses RFC 4648, section 4.
//     If the format is "base64url", then this uses RFC 4648, section 5.
//     If the format is "base32", then this uses RFC 4648, section 6.
//     If the format is "base32hex", then this uses RFC 4648, section 7.
//     If the format is "base16" or "hex", then this uses RFC 4648, section 8.
//     If the format is "array", then the bytes value is represented as a JSON array
//     where each element recursively uses the JSON representation of each byte.
//
//   - A Go float is usually represented as a JSON number.
//     If the format is "nonfinite", then NaN, +Inf, and -Inf are represented as
//     the JSON strings "NaN", "Infinity", and "-Infinity", respectively.
//     Without the use of this format, such string values result in a [SemanticError].
//
//   - A nil Go map is usually encoded using an empty JSON object.
//     If the format is "emitnull", then a nil map is encoded as a JSON null.
//     If the format is "emitempty", then a nil map is encoded as an empty JSON object,
//     regardless of whether [FormatNilMapAsNull] is specified.
//
//   - A nil Go slice is usually encoded using an empty JSON array.
//     If the format is "emitnull", then a nil slice is encoded as a JSON null.
//     If the format is "emitempty", then a nil slice is encoded as an empty JSON array,
//     regardless of whether [FormatNilSliceAsNull] is specified.
//
//   - A Go pointer usually uses the JSON representation of the underlying value.
//     The format is forwarded to the marshaling and unmarshaling of the underlying type.
//
//   - A Go [time.Time] is usually represented as a JSON string containing
//     the timestamp formatted in RFC 3339 with nanosecond precision.
//     If the format matches one of the format constants declared
//     in the time package (e.g., RFC1123), then that format is used.
//     If the format is "unix", "unixmilli", "unixmicro", or "unixnano",
//     then the timestamp is represented as a possibly fractional JSON number
//     of the number of seconds (or milliseconds, microseconds, or nanoseconds)
//     since the Unix epoch, which is January 1st, 1970 at 00:00:00 UTC.
//     To avoid a fractional component when encoding,
//     round the timestamp to the relevant unit.
//     Otherwise if non-empty, the format is used as-is and
//     encoded using [time.Time.Format] and
//     decoded using [time.Time.Parse].
//
//   - A Go [time.Duration] usually has no default representation.
//     If the format is "sec", "milli", "micro", or "nano",
//     then the duration is represented as a possibly fractional JSON number
//     of the number of seconds (or milliseconds, microseconds, or nanoseconds).
//     To avoid a fractional component when encoding,
//     round the duration to the relevant unit.
//     If the format is "units", it is represented as a JSON string
//     encoded using [time.Duration.String] and decoded using [time.ParseDuration]
//     (e.g., "1h30m" for 1 hour 30 minutes).
//     If the format is "iso8601", it is represented as a JSON string using the
//     ISO 8601 standard for durations (e.g., "PT1H30M" for 1 hour 30 minutes)
//     using only accurate units of hours, minutes, and seconds.
package json