Serializes the JsonDocument to create a prettified JSON document, i.e. a document with spaces and line-breaks between values.

If you want a “minified” JSON document, use serializeJson()


size_t serializeJsonPretty(const JsonDocument& doc, char* output, size_t outputSize);
size_t serializeJsonPretty(const JsonDocument& doc, char output[size]);
size_t serializeJsonPretty(const JsonDocument& doc, Print& output);
size_t serializeJsonPretty(const JsonDocument& doc, String& output);  // ⚠️ appends (see below)
size_t serializeJsonPretty(const JsonDocument& doc, std::string& output);  // ⚠️ appends (see below)

template<typename Writer> // custom writer class (see below)
size_t serializeJsonPretty(const JsonDocument& doc, Writer& output);

Note: the two first overloads support unsigned char as well.


  • doc: the JsonDocument to serialize (in fact, you can also use a JsonArray, a JsonObject, or a JsonVariant)
  • output: the destination where the JSON document should be written
  • outputSize: the capacity of the destination buffer

Because output can be any implementation of Print, you can uses instances like Serial, EthernetClient, WifiClient

This function treats String and std::string as streams: it doesn’t replace the content, it appends to the end.

Return value

The number of characters written.


You can configure serializeJsonPretty() with the following settings:

How to view the JSON output?

When you pass a Stream to serializeJsonPretty(), it writes the JSON to the stream but doesn’t print anything to the serial port, which makes troubleshooting difficult.

If you want to see what serializeJsonPretty() writes, use WriteLoggingStream from the StreamUtils library. See the example below.


When you pass a Stream to serializeJsonPretty(), it sends the bytes one by one, which can be slow depending on the target stream. For example, if you send to a WiFiClient on an ESP8266, it will send a packet over the air for each byte, which is terribly slow and inefficient. To improve speed and efficiency, we must send fewer, larger packets.

To write the JSON document in chunks, you can use WriteBufferingStream from the StreamUtils library. See the example in serializeJson()

Custom writer

If none of the supported output types is suitable for you, you can implement a custom writer class. This class must implement two member functions, as shown below:

struct CustomWriter {
  // Writes one byte, returns the number of bytes written (0 or 1)
  size_t write(uint8_t c);
  // Writes several bytes, returns the number of bytes written
  size_t write(const uint8_t *buffer, size_t length);

Then, pass a reference to an instance of this class as the second argument of serializeJsonPretty().


StaticJsonDocument<200> doc;
doc["hello"] = "world";
serializeJsonPretty(doc, Serial);

will write the following string to the serial output:

  "hello": "world"

See also