You can do this natively wihtout relying on IP services. You can get the user's timezone like this:

CopyIntl.DateTimeFormat().resolvedOptions().timeZone

and then extract the country from that value. Here is a working example on CodePen.

I have used the below code using jQuery and its working fine for me. I am getting the country_code , country_name etc.,.

 jQuery(document).ready(function($) {
    jQuery.getScript('http://www.geoplugin.net/javascript.gp', function() 
    {
        var country = geoplugin_countryName();
        alert(country);
        var code = geoplugin_countryCode();
        alert(code);
        console.log("Your location is: " + country + ", " + zone + ", " + district);
    });
});

Note: Do remember to import the plugin script as below:

<script src="http://www.geoplugin.net/javascript.gp" type="text/javascript"></script>

I think there is nothing like that build into the Javascript standard library, but as always, there is an NPM library for it: https://www.npmjs.com/package/i18n-iso-countries

There are multiple options to determine the locale. In descending order of usefulness, these are:

1. Look up the IP address, with an IP geolocation service like Maxmind GeoIP. This is the location the request is coming from, i.e. if an American vacations in Italy and uses a Swedish VPN, it will return Sweden.

It can only be done with the help of the server. The main advantage is that it's very reliable. The accuracy will be country or region for free services, city or region for paid ones.

2. Look up the precise location on Earth from the browser with the geolocation API. An American vacationing in Italy using a Swedish VPN will register as Italy.

The answer will be very precise, usually no more than 10m off. In principle, it could work client-side, although you may want to perform the coordinate -> country lookup on the server. The main disadvantages are that not all devices have either GPS or WiFi position, and that it generally requires explicit user consent.

3. Look in the Accept-Language header on the server (or with the help of the server), and extract the locale information. An American vacationing in Italy using a Swedish VPN will register as USA.

The downside is that this is a setting that's extremely easy to change. For instance, English speakers around the world may prefer en-US settings in order to avoid machine-translated text. On modern browsers (as of writing not IE/Edge, and only Safari 11+), you can also request navigator.languages.

4. navigator.language is the first element of the navigator.languages header. All of the considerations of navigator.languages apply. On top, this information can sometimes be just the language without any locale (i.e. en instead of en-US).

5. Use another, third-party service. For instance, if the user signs in via a Single-Sign-On system such Facebook connect, you can request the hometown of the user. This information is typically very unreliable, and requires a third party.

navigator.language isn't reliable as one of your linked questions states.

The reason this is asked a lot, but you're still searching says something about the problem. That language detection purely on the client side is not anything close to reliable.

First of all language preferences should only be used to detect language preferences - i.e. not location. My browser is set to en_US, because I wanted the English version. But I'm in the UK, so would have to alter this to en_GB to have my country detected via my browser settings. As the 'customer' that's not my problem. That's fine for language, but no good if all the prices on your site are in $USD.

To detect language you really do need access to a server side script. If you're not a back end dev and want to do as much as possible on the client side (as your question), all you need is a one line PHP script that echos back the Accept-Language header. At its simplest it could just be:

<?php
echo $_SERVER['HTTP_ACCEPT_LANGUAGE']; 
// e.g. "en-US,en;q=0.8"

You could get this via Ajax and parse the text response client side, e.g (using jQuery):

$.ajax( { url: 'script.php', success: function(raw){
    var prefs = raw.split(',');
    // process language codes ....
} } );

If you were able to generate your HTML via a back end, you could avoid using Ajax completely by simply printing the language preferences into your page, e.g.

<script>
    var prefs = <?php echo json_encode($_SERVER['HTTP_ACCEPT_LANGUAGE'])?>;
</script>

If you had no access to the server but could get a script onto another server, a simple JSONP service would look like:

<?php
$prefs = $_SERVER['HTTP_ACCEPT_LANGUAGE'];
$jsonp = 'myCallback('.json_encode($prefs).')';

header('Content-Type: application/json; charset=UTF-8', true );
header('Content-Length: '.strlen($jsonp), true );
echo $jsonp;

Using jQuery for your Ajax you'd do something like:

function myCallback( raw ){
    var prefs = raw.split(',');
    // process language codes ....
}
$.ajax( {
    url: 'http://some.domain/script.php',
    dataType: 'jsonp'
} );

Country detection is another matter. On the client side there is navigator.geolocation, but it will most likely prompt your user for permission, so no good for a seamless user experience.

To do invisibly, you're limited to geo IP detection. By the same token as above, don't use language to imply country either.

To do country detection on the client side, you'll also need a back end service in order to get the client IP address and access a database of IP/location mappings. Maxmind's GeoIP2 JavaScript client appears to wrap this all up in a client-side bundle for you, so you won't need your own server (although I'm sure it will use a remote jsonp service). There's also freegeoip.net, which is probably less hassle than MaxMind in terms of signing up, and it appears to be open source too.