runtime.Port
============

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.

| Connection type | Initiate connection attempt | Handle connection attempt |
| --- | --- | --- |
| Background script to content script | `tabs.connect()` | `runtime.onConnect` |
| Content script to background script | `runtime.connect()` | `runtime.onConnect` |
| Extension to native application | `runtime.connectNative()` | 
 Not applicable (see
 Native messaging).
  |
| Extension to Extension | `runtime.connect()` | `runtime.onConnectExternal` |

Type
----

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

`name`

`string`. The port's name, defined in the `runtime.connect()` or `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:


```js
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 `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

`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
---------------------

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

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.


```js
// 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.


```js
// 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.


```js
// 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:


```js
/\*
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");
});
```


**Note:** This API is based on Chromium's `chrome.runtime` API. This documentation is derived from `runtime.json` in the Chromium code.