How to know if browser tab is already open using Javascript?

You may use something like following

<!-- HTML -->
<a id="opener">Open window</a>

// JavaScript
var a = document.getElementById('opener'), w;        
a.onclick = function() {
  if (!w || w.closed) {
    w = window.open("https://www.google.com","_blank","menubar = 0, scrollbars = 0");
  } else {
    console.log('window is already opened');
  }
  w.focus();
};

Working jsBin | More on window.open method

If you want to control more than one window, use the snippet below

<!-- HTML -->
<a href="https://www.google.com" class="opener">Open google.com</a> | 
<a href="http://www.yahoo.com" class="opener">Open yahoo.com</a> 

//JavaScript
window.onload = function(){
  var a = document.querySelectorAll('.opener'), w = [], url, random, i;
  for(i = 0; i < a.length; i++){
    (function(i){
      a[i].onclick = function(e) {
        if (!w[i] || w[i].closed) {
          url = this.href;
          random = Math.floor((Math.random() * 100) + 1); 
          w[i] = window.open(url, "_blank", random, "menubar = 0, scrollbars = 0");
        } else {
          console.log('window ' + url + ' is already opened');
        }
        e.preventDefault();
        w[i].focus();
      };
    })(i);
  }
};

Working jsBin

If you don't want them to load in separated window, just exclude this line

random = Math.floor((Math.random()*100)+1);

and remove random reference from the next line

w[i] = window.open(url, "_blank", random, "menubar=0,scrollbars=0");

Side note: As you can see above, we created two windows with some third party content; you should know that there's no way to get any reference (to the parent/opener window) from them.

The answer by Casey Chu works fine until the browser crashes with the page open. On any next execution, the localStorage object will have initialized tabCount with non zero value. Therefore a better solution is to store the value in a session cookie. The session cookie will be removed when browser exits successfully. When the browser crashes the session cookie will actually be preserved but fortunately only for one next execution of the browser.

Object sessionStorage is distinct for each tab so it cannot be used for sharing tab count.

This is the improved solution using js-cookie library.

if (+Cookies.get('tabs') > 0)
    alert('Already open!');
else
    Cookies.set('tabs', 0);

Cookies.set('tabs', +Cookies.get('tabs') + 1);

window.onunload = function () {
        Cookies.set('tabs', +Cookies.get('tabs') - 1);
};

One basic idea is to store the tab count in either a cookie or localStorage, incrementing it on page load and decrementing it on page unload:

if (+localStorage.tabCount > 0)
    alert('Already open!');
else
    localStorage.tabCount = 0;

localStorage.tabCount = +localStorage.tabCount + 1;
window.onunload = function () {
    localStorage.tabCount = +localStorage.tabCount - 1;
};

Try opening this fiddle in multiple tabs.

Note that this technique is pretty fragile, though. For example, if for some reason the browser crashes, the unload handler won't run, and it'll go out of sync.