runtime.Port

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A Port object represents one end of a connection between two specific contexts, which can be used to exchange messages.

One side initiates the connection, using a connect() API. This returns a Port object. The other side listens for connection attempts using an onConnect listener. This is passed a corresponding Port object.

Once both sides have Port objects, they can exchange messages using Port.postMessage() and Port.onMessage. When they are finished, either end can disconnect using Port.disconnect(), which will generate a Port.onDisconnect event at the other end, enabling the other end to do any cleanup required.

A Port can also become disconnected in response to various events. See Lifecycle.

You can use this pattern to communicate between:

• different parts of your extension (for example, between content scripts and background scripts)
• between your extension and a native application running on the user’s computer.
• between your extension and a different extension

You need to use different connection APIs for different sorts of connections, as detailed in the table below.

Type

Values of this type are objects. They contain the following properties:

• name

  • : string. The port’s name, defined in the {{WebExtAPIRef("runtime.connect()")}}  or {{WebExtAPIRef("tabs.connect()")}}  call that created it. If this port is connected to a native application, its name is the name of the native application.

• disconnect

  • : function. Disconnects a port. Either end can call this when they have finished with the port. It will cause onDisconnect to be fired at the other end. This is useful if the other end is maintaining some state relating to this port, which can be cleaned up on disconnect. If this port is connected to a native application, this function will close the native application.

• error

  • : object. If the port was disconnected due to an error, this will be set to an object with a string property message, giving you more information about the error. See onDisconnect.

• onDisconnect

  • : object. This contains the addListener() and removeListener() functions common to all events for extensions built using WebExtension APIs. Listener functions will be called when the other end has called Port.disconnect(). This event will only be fired once for each port. The listener function will be passed the Port object. If the port was disconnected due to an error, then the Port argument will contain an error property giving more information about the error:

    port.onDisconnect.addListener((p) => {
      if (p.error) {
        console.log(`Disconnected due to an error: ${p.error.message}`);
      }
    });
    

    Note that in Google Chrome port.error is not supported: instead, use {{WebExtAPIRef("runtime.lastError")}}  to get the error message.

• onMessage

  • : object. This contains the addListener() and removeListener() functions common to all events for extensions built using WebExtension APIs. Listener functions will be called when the other end has sent this port a message. The listener will be passed the value that the other end sent.

• postMessage

  • : function. Send a message to the other end. This takes one argument, which is a serializable value (see Data cloning algorithm) representing the message to send. It will be delivered to any script listening to the port’s onMessage event, or to the native application if this port is connected to a native application.

• sender {{optional_inline}} 

  • : {{WebExtAPIRef('runtime.MessageSender')}} . Contains information about the sender of the message. This property will only be present on ports passed to onConnect/onConnectExternal listeners.

Lifecycle

The lifecycle of a Port is described in the Chrome docs.

There is, however, one important difference between Firefox and Chrome, stemming from the fact that the runtime.connect and tabs.connect APIs are broadcast channels. This means that there may be potentially more than one recipient, and this results in ambiguity when one of the contexts with a runtime.onConnect call is closed. In Chrome, a port stays active as long as there is any other recipient. In Firefox, the port closes when any of the contexts unloads. In other words, the disconnection condition,

• All frames that received the port (via runtime.onConnect) have unloaded.

which holds in Chrome, is replaced by

• Any frame that received the port (via runtime.onConnect) has unloaded.

in Firefox (see bug 1465514).

Browser compatibility

{{Compat}} 

Examples

Connecting from content scripts

This content script:

• connects to the background script and stores the Port in a variable called myPort.
• listens for messages on myPort and logs them.
• sends messages to the background script, using myPort, when the user clicks the document.

// content-script.js

let myPort = browser.runtime.connect({ name: "port-from-cs" });
myPort.postMessage({ greeting: "hello from content script" });

myPort.onMessage.addListener((m) => {
  console.log("In content script, received message from background script: ");
  console.log(m.greeting);
});

document.body.addEventListener("click", () => {
  myPort.postMessage({ greeting: "they clicked the page!" });
});

The corresponding background script:

• listens for connection attempts from the content script.

• when it receives a connection attempt:

  • stores the port in a variable named portFromCS.
  • sends the content script a message using the port.
  • starts listening to messages received on the port, and logs them.

• sends messages to the content script, using portFromCS, when the user clicks the extension’s browser action.

// background-script.js

let portFromCS;

function connected(p) {
  portFromCS = p;
  portFromCS.postMessage({ greeting: "hi there content script!" });
  portFromCS.onMessage.addListener((m) => {
    console.log("In background script, received message from content script");
    console.log(m.greeting);
  });
}

browser.runtime.onConnect.addListener(connected);

browser.browserAction.onClicked.addListener(() => {
  portFromCS.postMessage({ greeting: "they clicked the button!" });
});

Multiple content scripts

If you have multiple content scripts communicating at the same time, you might want to store each connection in an array.

// background-script.js

let ports = [];

function connected(p) {
  ports[p.sender.tab.id] = p;
  // …
}

browser.runtime.onConnect.addListener(connected);

browser.browserAction.onClicked.addListener(() => {
  ports.forEach((p) => {
    p.postMessage({ greeting: "they clicked the button!" });
  });
});

Connecting to native applications

This example connects to the native application “ping_pong” and starts listening for messages from it. It also sends the native application a message when the user clicks a browser action icon:

/*
On startup, connect to the "ping_pong" app.
*/
let port = browser.runtime.connectNative("ping_pong");

/*
Listen for messages from the app.
*/
port.onMessage.addListener((response) => {
  console.log(`Received: ${response}`);
});

/*
On a click on the browser action, send the app a message.
*/
browser.browserAction.onClicked.addListener(() => {
  console.log("Sending:  ping");
  port.postMessage("ping");
});

{{WebExtExamples}} 

[!NOTE] This API is based on Chromium’s chrome.runtime API. This documentation is derived from runtime.json in the Chromium code.