json

func Compact ¶

func Compact(dst *bytes.Buffer, src []byte) error

Compact appends to dst the JSON-encoded src with insignificant space characters elided.

func HTMLEscape ¶

func HTMLEscape(dst *bytes.Buffer, src []byte)

HTMLEscape appends to dst the JSON-encoded src with <, >, &, U+2028 and U+2029 characters inside string literals changed to \u003c, \u003e, \u0026, \u2028, \u2029 so that the JSON will be safe to embed inside HTML <script> tags. For historical reasons, web browsers don't honor standard HTML escaping within <script> tags, so an alternative JSON encoding must be used.

var out bytes.Buffer
json.HTMLEscape(&out, []byte(`{"Name":"<b>HTML content</b>"}`))
out.WriteTo(os.Stdout)

Output:

{"Name":"\u003cb\u003eHTML content\u003c/b\u003e"}

func Indent ¶

func Indent(dst *bytes.Buffer, src []byte, prefix, indent string) error

Indent appends to dst an indented form of the JSON-encoded src. Each element in a JSON object or array begins on a new, indented line beginning with prefix followed by one or more copies of indent according to the indentation nesting. The data appended to dst does not begin with the prefix nor any indentation, to make it easier to embed inside other formatted JSON data. Although leading space characters (space, tab, carriage return, newline) at the beginning of src are dropped, trailing space characters at the end of src are preserved and copied to dst. For example, if src has no trailing spaces, neither will dst; if src ends in a trailing newline, so will dst.

type Road struct {
	Name   string
	Number int
}
roads := []Road{
	{"Diamond Fork", 29},
	{"Sheep Creek", 51},
}

b, err := json.Marshal(roads)
if err != nil {
	log.Fatal(err)
}

var out bytes.Buffer
json.Indent(&out, b, "=", "\t")
out.WriteTo(os.Stdout)

Output:

[
=	{
=		"Name": "Diamond Fork",
=		"Number": 29
=	},
=	{
=		"Name": "Sheep Creek",
=		"Number": 51
=	}
=]

func Marshal ¶

func Marshal(v any) ([]byte, error)

Marshal returns the JSON encoding of v.

Marshal traverses the value v recursively. If an encountered value implements Marshaler and is not a nil pointer, Marshal calls [Marshaler.MarshalJSON] to produce JSON. If no [Marshaler.MarshalJSON] method is present but the value implements encoding.TextMarshaler instead, Marshal calls encoding.TextMarshaler.MarshalText and encodes the result as a JSON string. The nil pointer exception is not strictly necessary but mimics a similar, necessary exception in the behavior of [Unmarshaler.UnmarshalJSON].

Otherwise, Marshal uses the following type-dependent default encodings:

Boolean values encode as JSON booleans.

Floating point, integer, and Number values encode as JSON numbers. NaN and +/-Inf values will return an UnsupportedValueError.

String values encode as JSON strings coerced to valid UTF-8, replacing invalid bytes with the Unicode replacement rune. So that the JSON will be safe to embed inside HTML <script> tags, the string is encoded using HTMLEscape, which replaces "<", ">", "&", U+2028, and U+2029 are escaped to "\u003c","\u003e", "\u0026", "\u2028", and "\u2029". This replacement can be disabled when using an Encoder, by calling Encoder.SetEscapeHTML(false).

Array and slice values encode as JSON arrays, except that []byte encodes as a base64-encoded string, and a nil slice encodes as the null JSON value.

Struct values encode as JSON objects. Each exported struct field becomes a member of the object, using the field name as the object key, unless the field is omitted for one of the reasons given below.

The encoding of each struct field can be customized by the format string stored under the "json" key in the struct field's tag. The format string gives the name of the field, possibly followed by a comma-separated list of options. The name may be empty in order to specify options without overriding the default field name.

The "omitempty" option specifies that the field should be omitted from the encoding if the field has an empty value, defined as false, 0, a nil pointer, a nil interface value, and any array, slice, map, or string of length zero.

As a special case, if the field tag is "-", the field is always omitted. Note that a field with name "-" can still be generated using the tag "-,".

Examples of struct field tags and their meanings:

// Field appears in JSON as key "myName".
Field int `json:"myName"`

// Field appears in JSON as key "myName" and
// the field is omitted from the object if its value is empty,
// as defined above.
Field int `json:"myName,omitempty"`

// Field appears in JSON as key "Field" (the default), but
// the field is skipped if empty.
// Note the leading comma.
Field int `json:",omitempty"`

// Field is ignored by this package.
Field int `json:"-"`

// Field appears in JSON as key "-".
Field int `json:"-,"`

The "omitzero" option specifies that the field should be omitted from the encoding if the field has a zero value, according to rules:

1) If the field type has an "IsZero() bool" method, that will be used to determine whether the value is zero.

2) Otherwise, the value is zero if it is the zero value for its type.

If both "omitempty" and "omitzero" are specified, the field will be omitted if the value is either empty or zero (or both).

The "string" option signals that a field is stored as JSON inside a JSON-encoded string. It applies only to fields of string, floating point, integer, or boolean types. This extra level of encoding is sometimes used when communicating with JavaScript programs:

Int64String int64 `json:",string"`

The key name will be used if it's a non-empty string consisting of only Unicode letters, digits, and ASCII punctuation except quotation marks, backslash, and comma.

Embedded struct fields are usually marshaled as if their inner exported fields were fields in the outer struct, subject to the usual Go visibility rules amended as described in the next paragraph. An anonymous struct field with a name given in its JSON tag is treated as having that name, rather than being anonymous. An anonymous struct field of interface type is treated the same as having that type as its name, rather than being anonymous.

The Go visibility rules for struct fields are amended for JSON when deciding which field to marshal or unmarshal. If there are multiple fields at the same level, and that level is the least nested (and would therefore be the nesting level selected by the usual Go rules), the following extra rules apply:

1) Of those fields, if any are JSON-tagged, only tagged fields are considered, even if there are multiple untagged fields that would otherwise conflict.

2) If there is exactly one field (tagged or not according to the first rule), that is selected.

3) Otherwise there are multiple fields, and all are ignored; no error occurs.

Handling of anonymous struct fields is new in Go 1.1. Prior to Go 1.1, anonymous struct fields were ignored. To force ignoring of an anonymous struct field in both current and earlier versions, give the field a JSON tag of "-".

Map values encode as JSON objects. The map's key type must either be a string, an integer type, or implement encoding.TextMarshaler. The map keys are sorted and used as JSON object keys by applying the following rules, subject to the UTF-8 coercion described for string values above:

• keys of any string type are used directly
• keys that implement encoding.TextMarshaler are marshaled
• integer keys are converted to strings

Pointer values encode as the value pointed to. A nil pointer encodes as the null JSON value.

Interface values encode as the value contained in the interface. A nil interface value encodes as the null JSON value.

Channel, complex, and function values cannot be encoded in JSON. Attempting to encode such a value causes Marshal to return an UnsupportedTypeError.

JSON cannot represent cyclic data structures and Marshal does not handle them. Passing cyclic structures to Marshal will result in an error.

type ColorGroup struct {
	ID     int
	Name   string
	Colors []string
}
group := ColorGroup{
	ID:     1,
	Name:   "Reds",
	Colors: []string{"Crimson", "Red", "Ruby", "Maroon"},
}
b, err := json.Marshal(group)
if err != nil {
	fmt.Println("error:", err)
}
os.Stdout.Write(b)

Output:

{"ID":1,"Name":"Reds","Colors":["Crimson","Red","Ruby","Maroon"]}

func MarshalIndent ¶

func MarshalIndent(v any, prefix, indent string) ([]byte, error)

MarshalIndent is like Marshal but applies Indent to format the output. Each JSON element in the output will begin on a new line beginning with prefix followed by one or more copies of indent according to the indentation nesting.

data := map[string]int{
	"a": 1,
	"b": 2,
}

b, err := json.MarshalIndent(data, "<prefix>", "<indent>")
if err != nil {
	log.Fatal(err)
}

fmt.Println(string(b))

Output:

{
<prefix><indent>"a": 1,
<prefix><indent>"b": 2
<prefix>}

func Unmarshal ¶

func Unmarshal(data []byte, v any) error

Unmarshal parses the JSON-encoded data and stores the result in the value pointed to by v. If v is nil or not a pointer, Unmarshal returns an InvalidUnmarshalError.

Unmarshal uses the inverse of the encodings that Marshal uses, allocating maps, slices, and pointers as necessary, with the following additional rules:

To unmarshal JSON into a pointer, Unmarshal first handles the case of the JSON being the JSON literal null. In that case, Unmarshal sets the pointer to nil. Otherwise, Unmarshal unmarshals the JSON into the value pointed at by the pointer. If the pointer is nil, Unmarshal allocates a new value for it to point to.

To unmarshal JSON into a value implementing Unmarshaler, Unmarshal calls that value's [Unmarshaler.UnmarshalJSON] method, including when the input is a JSON null. Otherwise, if the value implements encoding.TextUnmarshaler and the input is a JSON quoted string, Unmarshal calls encoding.TextUnmarshaler.UnmarshalText with the unquoted form of the string.

To unmarshal JSON into a struct, Unmarshal matches incoming object keys to the keys used by Marshal (either the struct field name or its tag), preferring an exact match but also accepting a case-insensitive match. By default, object keys which don't have a corresponding struct field are ignored (see Decoder.DisallowUnknownFields for an alternative).

To unmarshal JSON into an interface value, Unmarshal stores one of these in the interface value:

• bool, for JSON booleans
• float64, for JSON numbers
• string, for JSON strings
• []any, for JSON arrays
• map[string]any, for JSON objects
• nil for JSON null

To unmarshal a JSON array into a slice, Unmarshal resets the slice length to zero and then appends each element to the slice. As a special case, to unmarshal an empty JSON array into a slice, Unmarshal replaces the slice with a new empty slice.

To unmarshal a JSON array into a Go array, Unmarshal decodes JSON array elements into corresponding Go array elements. If the Go array is smaller than the JSON array, the additional JSON array elements are discarded. If the JSON array is smaller than the Go array, the additional Go array elements are set to zero values.

To unmarshal a JSON object into a map, Unmarshal first establishes a map to use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal reuses the existing map, keeping existing entries. Unmarshal then stores key-value pairs from the JSON object into the map. The map's key type must either be any string type, an integer, or implement encoding.TextUnmarshaler.

If the JSON-encoded data contain a syntax error, Unmarshal returns a SyntaxError.

If a JSON value is not appropriate for a given target type, or if a JSON number overflows the target type, Unmarshal skips that field and completes the unmarshaling as best it can. If no more serious errors are encountered, Unmarshal returns an UnmarshalTypeError describing the earliest such error. In any case, it's not guaranteed that all the remaining fields following the problematic one will be unmarshaled into the target object.

The JSON null value unmarshals into an interface, map, pointer, or slice by setting that Go value to nil. Because null is often used in JSON to mean “not present,” unmarshaling a JSON null into any other Go type has no effect on the value and produces no error.

When unmarshaling quoted strings, invalid UTF-8 or invalid UTF-16 surrogate pairs are not treated as an error. Instead, they are replaced by the Unicode replacement character U+FFFD.

var jsonBlob = []byte(`[
	{"Name": "Platypus", "Order": "Monotremata"},
	{"Name": "Quoll",    "Order": "Dasyuromorphia"}
]`)
type Animal struct {
	Name  string
	Order string
}
var animals []Animal
err := json.Unmarshal(jsonBlob, &animals)
if err != nil {
	fmt.Println("error:", err)
}
fmt.Printf("%+v", animals)

Output:

[{Name:Platypus Order:Monotremata} {Name:Quoll Order:Dasyuromorphia}]

func Valid ¶

func Valid(data []byte) bool

Valid reports whether data is a valid JSON encoding.

goodJSON := `{"example": 1}`
badJSON := `{"example":2:]}}`

fmt.Println(json.Valid([]byte(goodJSON)), json.Valid([]byte(badJSON)))

Output:

true false