Obtaining "this" tab ID from content script in Chrome extension?

Tab id is automatically passed inside MessageSender object:

chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) {
    console.log("sent from tab.id=", sender.tab.id);
});

Note: According to docs, this property is not always available:

This property will only be present when the connection was opened from a tab (including content scripts), and only if the receiver is an extension, not an app.

Correction of the selected answer for nowadays:

chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) {
    console.log("sent from tab.id=", sender.id);
});

If you want the tabId of yourself (in my case in Content Script) without requiring "tabs" permission, a workaround is to have Content Script send a dummy message to the background script, then have background script respond with sender.tab.id back to the Content Script!

e.g. in content.js:

chrome.runtime.sendMessage({ text: "what is my tab_id?" }, tabId => {
   console.log('My tabId is', tabId);
});

and in background.js:

chrome.runtime.onMessage.addListener(function(msg, sender, sendResponse) {
    if (msg.text == "what is my tab_id?") {
        sendResponse({tab: sender.tab.id});
     }
});

it's a stupid workaround that worked for me. :)

PS. Oh, if you have tabs permission then you can run this async query very easily:

chrome.tabs.query({active: true, currentWindow: true}, function(tabs){
    var myTabId = tabs[0].id;
    chrome.tabs.sendMessage(myTabId, {text: "hi"}, function(response) {
        alert(response);
    });
});

If you're using Ports for two-way long-lived connections, the second argument in the callback is a Port object, so to access the tab ID is then:

chrome.runtime.onConnect.addListener(port => {
  if (port.name === "foo") {
    port.onMessage.addListener((msg, sendingPort) => {
      console.log("sent from tab.id=", sendingPort.sender.tab.id);
    });
  }
});