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A library for p5.js which adds support for interacting with Serial devices, using the Web Serial API (currently supported on Chrome and Edge).

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A library for p5.js which adds support for interacting with Serial devices, using the Web Serial API (currently supported on Chrome and Edge). It provides the following features:

  • Easy to use API, largely the same as Processing's Serial library
  • No async/await or callbacks needed in sketches
  • Can automatically connect to previously-used serial ports (great for installations)
  • Unicode support (Serial.print("你好"") in Arduino)
  • Multi-byte matching in readUntil(needle)
  • Well tested, also works in the p5.js web editor
  • Also supported on Chrome for Android


Getting started

Download the library file and include it in the head section of your HTML below the line that loads p5.js - or simply include the online version at the same place:

<script src="^1/libraries/p5.webserial.js"></script>


<script src="p5.webserial.js"></script>

Opening ports

Create a global variable, and set it to a new serial port instance inside setup:

let port;

function setup() {
  port = createSerial();
  // ...

To actually open a serial port, call the open method with the desired arguments. This prompts the user to select a serial port (at 9600 baud):;

This will only show Arduino boards (and compatible) in the dialog: (Other presets are MicroPython, RaspberryPi, Adafruit)'Arduino', 9600);

Most browsers will only show the port picker dialog as a result of user input, e.g. after clicking a button, so you likely will need to do this outside of setup. (see this example for how)

If the user has previously selected a serial port on a page, you can automatically connect to it on future page loads without user interaction, even inside setup, like so:

let usedPorts = usedSerialPorts();
if (usedPorts.length > 0) {[0], 9600);

Reading data

This reads a single (Unicode) character from the serial port:

let str =;                   // returns e.g. "你"

This reads all available characters:

let str =;                    // returns e.g. "你好"

This reads all characters till the end of a line: (This will return an empty string if the string given as parameter was not found.)

let str = port.readUntil("\n");           // returns the whole line

This also works with more than one character to look for:

let str = port.readUntil("STOP");         // returns everything up to and including "STOP"

This returns the most reccently returned character, discarding all previously received ones in the process:

let str = port.last();

These methods allow you to receive (raw) bytes as values from 0 to 255 instead of characters:

let num = port.readByte();                // returns a single byte, e.g. 72
let arr = port.readBytes(2);              // returns two bytes in an array, e.g. [ 72, 69 ]
let arr = port.readBytes();               // returns all bytes in an array, e.g. [ 72, 69, ..]
let arr = port.readBytesUntil(10);        // returns all bytes till value 10 in an array
let arr = port.readBytesUntil([13, 10]);  // returns all bytes till value 13 followed by 10
let num = port.lastByte();                // returns a single byte, e.g. 10

To find out how many characters (or bytes) are available to be read immediately:

let characters = port.available();        // how many characters
let bytes = port.availableBytes();        // how many bytes

Writing data

To send "HELLO" over the serial port:


To send the value 72 as a sequence of digits (the characters "7" and "2"): (you want to do this most of the time)


To send a single byte with the value 72:


To send a series of bytes:

port.write([72, 69, 76, 76, 79]);


To check if the serial port is open:

if (port.opened()) {
  // the port is indeed open

To close the port:


To clear everything in the input buffer:


To setting the DTR (Data Terminal Ready) or RTS (Request to Send) lines:

port.dtr(true); // or port.dtr(false)
port.rts(true); // or port.rts(false)

Resetting a connected Arduino Uno microcontroller e.g. works with:

setTimeout(function() {
}, 200);


  • WebSerial might not work on sites served over the insecure http:// protocol, so try to use a server that uses https:// instead. (Presently, localhost works fine over http on Chrome however.)


A library for p5.js which adds support for interacting with Serial devices, using the Web Serial API (currently supported on Chrome and Edge).






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