Message Passing

Since content scripts run in the context of a web page and not the extension, they often need some way of communicating with the rest of the extension. For example, an RSS reader extension might use content scripts to detect the presence of an RSS feed on a page, then notify the background page in order to display a page action icon for that page.

Communication between extensions and their content scripts works by using message passing. Either side can listen for messages sent from the other end, and respond on the same channel. A message can contain any valid JSON object (null, boolean, number, string, array, or object). There is a simple API for one-time requests and a more complex API that allows you to have long-lived connections for exchanging multiple messages with a shared context. It is also possible to send a message to another extension if you know its ID, which is covered in the cross-extension messages section.

Simple one-time requests

If you only need to send a single message to another part of your extension (and optionally get a response back), you should use the simplified runtime.sendMessage or tabs.sendMessage methods. This lets you send a one-time JSON-serializable message from a content script to extension, or vice versa, respectively. An optional callback parameter allows you handle the response from the other side, if there is one.

Sending a request from a content script looks like this:

chrome.runtime.sendMessage({greeting: "hello"}, function(response) {
  console.log(response.farewell);
});

Sending a request from the extension to a content script looks very similar, except that you need to specify which tab to send it to. This example demonstrates sending a message to the content script in the selected tab.

chrome.tabs.query({active: true, currentWindow: true}, function(tabs) {
  chrome.tabs.sendMessage(tabs[0].id, {greeting: "hello"}, function(response) {
    console.log(response.farewell);
  });
});

On the receiving end, you need to set up an runtime.onMessage event listener to handle the message. This looks the same from a content script or extension page.

chrome.runtime.onMessage.addListener(
  function(request, sender, sendResponse) {
    console.log(sender.tab ?
                "from a content script:" + sender.tab.url :
                "from the extension");
    if (request.greeting == "hello")
      sendResponse({farewell: "goodbye"});
  });

Note: If multiple pages are listening for onMessage events, only the first to call sendResponse() for a particular event will succeed in sending the response. All other responses to that event will be ignored.

Long-lived connections

Sometimes it's useful to have a conversation that lasts longer than a single request and response. In this case, you can open a long-lived channel from your content script to an extension page, or vice versa, using runtime.connect or tabs.connect respectively. The channel can optionally have a name, allowing you to distinguish between different types of connections.

One use case might be an automatic form fill extension. The content script could open a channel to the extension page for a particular login, and send a message to the extension for each input element on the page to request the form data to fill in. The shared connection allows the extension to keep shared state linking the several messages coming from the content script.

When establishing a connection, each end is given a runtime.Port object which is used for sending and receiving messages through that connection.

Here is how you open a channel from a content script, and send and listen for messages:

var port = chrome.runtime.connect({name: "knockknock"});
port.postMessage({joke: "Knock knock"});
port.onMessage.addListener(function(msg) {
  if (msg.question == "Who's there?")
    port.postMessage({answer: "Madame"});
  else if (msg.question == "Madame who?")
    port.postMessage({answer: "Madame... Bovary"});
});

Sending a request from the extension to a content script looks very similar, except that you need to specify which tab to connect to. Simply replace the call to connect in the above example with tabs.connect.

In order to handle incoming connections, you need to set up a runtime.onConnect event listener. This looks the same from a content script or an extension page. When another part of your extension calls "connect()", this event is fired, along with the runtime.Port object you can use to send and receive messages through the connection. Here's what it looks like to respond to incoming connections:

chrome.runtime.onConnect.addListener(function(port) {
  console.assert(port.name == "knockknock");
  port.onMessage.addListener(function(msg) {
    if (msg.joke == "Knock knock")
      port.postMessage({question: "Who's there?"});
    else if (msg.answer == "Madame")
      port.postMessage({question: "Madame who?"});
    else if (msg.answer == "Madame... Bovary")
      port.postMessage({question: "I don't get it."});
  });
});

You may want to find out when a connection is closed, for example if you are maintaining separate state for each open port. For this you can listen to the runtime.Port.onDisconnect event. This event is fired either when the other side of the channel manually calls runtime.Port.disconnect, or when the page containing the port is unloaded (for example if the tab is navigated). onDisconnect is guaranteed to be fired only once for any given port.

Cross-extension messaging

In addition to sending messages between different components in your extension, you can use the messaging API to communicate with other extensions. This lets you expose a public API that other extensions can take advantage of.

Listening for incoming requests and connections is similar to the internal case, except you use the runtime.onMessageExternal or runtime.onConnectExternal methods. Here's an example of each:

// For simple requests:
chrome.runtime.onMessageExternal.addListener(
  function(request, sender, sendResponse) {
    if (sender.id == blacklistedExtension)
      return;  // don't allow this extension access
    else if (request.getTargetData)
      sendResponse({targetData: targetData});
    else if (request.activateLasers) {
      var success = activateLasers();
      sendResponse({activateLasers: success});
    }
  });

// For long-lived connections:
chrome.runtime.onConnectExternal.addListener(function(port) {
  port.onMessage.addListener(function(msg) {
    // See other examples for sample onMessage handlers.
  });
});

Likewise, sending a message to another extension is similar to sending one within your extension. The only difference is that you must pass the ID of the extension you want to communicate with. For example:

// The ID of the extension we want to talk to.
var laserExtensionId = "abcdefghijklmnoabcdefhijklmnoabc";

// Make a simple request:
chrome.runtime.sendMessage(laserExtensionId, {getTargetData: true},
  function(response) {
    if (targetInRange(response.targetData))
      chrome.runtime.sendMessage(laserExtensionId, {activateLasers: true});
  });

// Start a long-running conversation:
var port = chrome.runtime.connect(laserExtensionId);
port.postMessage(...);

Sending messages from web pages

Similar to cross-extension messaging, your app or extension can receive and respond to messages from regular web pages. To use this feature, you must first specify in your manifest.json which web sites you want to communicate with. For example:

"externally_connectable": {
  "matches": ["*://*.example.com/*"]
}

This will expose the messaging API to any page which matches the URL patterns you specify. The URL pattern must contain at least a second-level domain - that is, hostname patterns like "*", "*.com", "*.co.uk", and "*.appspot.com" are prohibited. From the web page, use the runtime.sendMessage or runtime.connect APIs to send a message to a specific app or extension. For example:

// The ID of the extension we want to talk to.
var editorExtensionId = "abcdefghijklmnoabcdefhijklmnoabc";

// Make a simple request:
chrome.runtime.sendMessage(editorExtensionId, {openUrlInEditor: url},
  function(response) {
    if (!response.success)
      handleError(url);
  });

From your app or extension, you may listen to messages from web pages via the runtime.onMessageExternal or runtime.onConnectExternal APIs, similar to cross-extension messaging. Only the web page can initiate a connection. Here is an example:

chrome.runtime.onMessageExternal.addListener(
  function(request, sender, sendResponse) {
    if (sender.url == blacklistedWebsite)
      return;  // don't allow this web page access
    if (request.openUrlInEditor)
      openUrl(request.openUrlInEditor);
  });

Native messaging

Extensions can exchange messages with native applications. Native applications that support this feature must register a native messaging host that knows how to communicate with the extension. Chrome starts the host in a separate process and communicates with it using standard input and standard output streams.

Native messaging host

In order to register a native messaging host the application must install a manifest file that defines the native messaging host configuration. Below is an example of the manifest file:

{
  "name": "com.my_company.my_application",
  "description": "My Application",
  "path": "C:\\Program Files\\My Application\\chrome_native_messaging_host.exe",
  "type": "stdio",
  "allowed_origins": [
    "chrome-extension://knldjmfmopnpolahpmmgbagdohdnhkik/"
  ]
}

Native messaging host manifest file contains the following fields:

Name	Description
name	Name of the native messaging host. Clients pass this string to runtime.connectNative or runtime.sendNativeMessage.
description	Short application description.
path	Path to the native messaging host binary. On Linux and OSX the path must be absolute. On Windows it can be relative to the directory in which the manifest file is located.
type	Type of the interface used to communicate with the native messaging host. Currently there is only one possible value for this parameter: stdio. It indicates that Chrome should use stdin and stdout to communicate with the host.
allowed_origins	List of extensions that should have access to the native messaging host.

Location of the manifest file depends on the platform:

Windows:

The manifest file can be located anywhere in the file system. The application installer must create registry key HKEY_LOCAL_MACHINE\SOFTWARE\Google\Chrome\NativeMessagingHosts\com.my_company.my_application or HKEY_CURRENT_USER\SOFTWARE\Google\Chrome\NativeMessagingHosts\com.my_company.my_application, and set default value of that key to the full path to the manifest file.

OSX:

The manifest file must be placed at /Library/Google/Chrome/NativeMessagingHosts/com.my_company.my_application.json, or, for applications installed on user level, ~/Library/Application Support/Google/Chrome/NativeMessagingHosts/com.my_company.my_application.json.

Linux:

The manifest file must be placed at /etc/opt/chrome/native-messaging-hosts/com.my_company.my_application.json, or, for applications installed on user level, ~/.config/google-chrome/NativeMessagingHosts/com.my_company.my_application.json.

Chrome starts each native messaging host in a separate process and communicates with it using standard input (stdin) and standard output (stdout). The same format is used to send messages in both directions: each message is serialized using JSON, UTF-8 encoded and is preceded with 32-bit message length in native byte order.

When a messaging port is created using runtime.connectNative Chrome starts native messaging host process and keeps it running until the port is destroyed. On the other hand, when a message is sent using runtime.sendNativeMessage, without creating a messaging port, Chrome starts a new native messaging host process for each message. In that case the first message generated by the host process is handled as a response to the original request, i.e. Chrome will pass it to the response callback specified when runtime.sendNativeMessage is called. All other messages generated by the native messaging host in that case are ignored.

Connecting to a native application

Sending and receiving messages to and from a native application is very similar to cross-extension messaging. The main difference is that runtime.connectNative is used instead of runtime.connect, and runtime.sendNativeMessage is used instead of runtime.sendMessage.

The Following example creates a runtime.Port object that's connected to native messaging host com.my_company.my_application, starts listening for messages from that port and sends one outgoing message:

var port = chrome.runtime.connectNative('com.my_company.my_application');
port.onMessage.addListener(function(msg) {
  console.log("Received" + msg);
});
port.onDisconnect.addListener(function() {
  console.log("Disconnected");
});
port.postMessage({ text: "Hello, my_application" });

runtime.sendNativeMessage can be used to send a message to native application without creating a port, e.g.:

chrome.runtime.sendNativeMessage('com.my_company.my_application',
  { text: "Hello" },
  function(response) {
    console.log("Received " + response);
  });

Security considerations

When receiving a message from a content script or another extension, your background page should be careful not to fall victim to cross-site scripting. Specifically, avoid using dangerous APIs such as the below:

chrome.tabs.sendMessage(tab.id, {greeting: "hello"}, function(response) {
  // WARNING! Might be evaluating an evil script!
  var resp = eval("(" + response.farewell + ")");
});

chrome.tabs.sendMessage(tab.id, {greeting: "hello"}, function(response) {
  // WARNING! Might be injecting a malicious script!
  document.getElementById("resp").innerHTML = response.farewell;
});

Instead, prefer safer APIs that do not run scripts:

chrome.tabs.sendMessage(tab.id, {greeting: "hello"}, function(response) {
  // JSON.parse does not evaluate the attacker's scripts.
  var resp = JSON.parse(response.farewell);
});

chrome.tabs.sendMessage(tab.id, {greeting: "hello"}, function(response) {
  // innerText does not let the attacker inject HTML elements.
  document.getElementById("resp").innerText = response.farewell;
});

Examples

You can find simple examples of communication via messages in the examples/api/messaging directory. examples/api/nativeMessaging contains an example application that uses native messaging. Also see the contentscript_xhr example, in which a content script and its parent extension exchange messages, so that the parent extension can perform cross-site requests on behalf of the content script. For more examples and for help in viewing the source code, see Samples.