webextensions-examples/native-messaging/add-on/background.js

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

/*
Listen for messages from the app and log them to the console.
*/
port.onMessage.addListener((response) => {
  console.log("Received: " + response);
});

/*
Listen for the native messaging port closing.
*/
port.onDisconnect.addListener((port) => {
  if (port.error) {
    console.log(`Disconnected due to an error: ${port.error.message}`);
  } else {
    // The port closed for an unspecified reason. If this occurred right after
    // calling `browser.runtime.connectNative()` there may have been a problem
    // starting the the native messaging client in the first place.
    console.log(`Disconnected`, port);
  }
});

/*
When the extension's action icon is clicked, send the app a message.
*/
browser.browserAction.onClicked.addListener(() => {
  console.log("Sending:  ping");
  port.postMessage("ping");
});