WebSocket

The WebSocket object provides the API for creating and managing a WebSocket connection to a server, as well as for sending and receiving data on the connection.

The WebSocket constructor accepts one required and one optional parameter:

WebSocket WebSocket(
  in DOMString url,
  in optional DOMString protocols
);

WebSocket WebSocket(
  in DOMString url,
  in optional DOMString[] protocols
);

url

The URL to which to connect; this should be the URL to which the WebSocket server will respond.

protocols Optional

Either a single protocol string or an array of protocol strings. These strings are used to indicate sub-protocols, so that a single server can implement multiple WebSocket sub-protocols (for example, you might want one server to be able to handle different types of interactions depending on the specified protocol). If you don't specify a protocol string, an empty string is assumed.

The constructor can throw exceptions:

SECURITY_ERR

The port to which the connection is being attempted is being blocked.

Method overview

void close(in optional unsigned long code, in optional DOMString reason);

void send(in DOMString data);

Attributes

Attribute	Type	Description
binaryType	DOMString	A string indicating the type of binary data being transmitted by the connection. This should be either "blob" if DOM Blob objects are being used or "arraybuffer" if ArrayBuffer objects are being used.
bufferedAmount	unsigned long	The number of bytes of data that have been queued using calls to send() but not yet transmitted to the network. This value resets to zero once all queued data has been sent. This value does not reset to zero when the connection is closed; if you keep calling send(), this will continue to climb. Read only
extensions	DOMString	The extensions selected by the server. This is currently only the empty string or a list of extensions as negotiated by the connection.
onclose	EventListener	An event listener to be called when the WebSocket connection's readyState changes to CLOSED. The listener receives a CloseEvent named "close".
onerror	EventListener	An event listener to be called when an error occurs. This is a simple event named "error".
onmessage	EventListener	An event listener to be called when a message is received from the server. The listener receives a MessageEvent named "message".
onopen	EventListener	An event listener to be called when the WebSocket connection's readyState changes to OPEN; this indicates that the connection is ready to send and receive data. The event is a simple one with the name "open".
protocol	DOMString	A string indicating the name of the sub-protocol the server selected; this will be one of the strings specified in the protocols parameter when creating the WebSocket object.
readyState	unsigned short	The current state of the connection; this is one of the Ready state constants. Read only.
url	DOMString	The URL as resolved by the constructor. This is always an absolute URL. Read only.

Constants

Ready state constants

These constants are used by the readyState attribute to describe the state of the WebSocket connection.

Constant	Value	Description
CONNECTING	0	The connection is not yet open.
OPEN	1	The connection is open and ready to communicate.
CLOSING	2	The connection is in the process of closing.
CLOSED	3	The connection is closed or couldn't be opened.

Methods

close()

Closes the WebSocket connection or connection attempt, if any. If the connection is already CLOSED, this method does nothing.

void close(
  in optional unsigned short code,
  in optional DOMString reason
);

Parameters

code Optional

A numeric value indicating the status code explaining why the connection is being closed. If this parameter is not specified, a default value of 1000 (indicating a normal "transaction complete" closure) is assumed. See the list of status codes on the CloseEvent page for permitted values.

reason Optional

A human-readable string explaining why the connection is closing. This string must be no longer than 123 bytes of UTF-8 text (not characters).

Exceptions thrown

INVALID_ACCESS_ERR

An invalid code was specified.

SYNTAX_ERR

The reason string is too long or contains unpaired surrogates.

send()

Transmits data to the server over the WebSocket connection.

void send(
  in DOMString data
);

void send(
  in ArrayBuffer data
);

void send(
  in Blob data
); 

Parameters

data

A text string to send to the server.

Exceptions thrown

INVALID_STATE_ERR

The connection is not currently OPEN.

SYNTAX_ERR

The data is a string that has unpaired surrogates.

Example

// Create WebSocket connection.
const socket = new WebSocket('ws://localhost:8080');

// Connection opened
socket.addEventListener('open', function (event) {
    socket.send('Hello Server!');
});

// Listen for messages
socket.addEventListener('message', function (event) {
    console.log('Message from server', event.data);
});

Specifications

Specification	Status	Comment
The WebSocket API The definition of 'WebSocket' in that specification.	Candidate Recommendation	Initial definition

Browser compatibility

[1] Starting in Gecko 6.0 (Firefox 6.0 / Thunderbird 6.0 / SeaMonkey 2.3), the constructor is prefixed; you will need to use MozWebSocket(): var mySocket = new MozWebSocket("http://www.example.com/socketserver");

The extensions attribute was not supported in Gecko until Gecko 8.0.

Prior to Gecko 11.0 (Firefox 11.0 / Thunderbird 11.0 / SeaMonkey 2.8), outbound messages sent using the send() method were limited to 16 MB. They can now be up to 2 GB in size.

See also

• Writing WebSocket client applications