When the page is reloaded, totalSubs returns back to its original value of 10. Why is this?

var totalSubs = 10;
window.onload = function getSubs() {
document.getElementById('subsNo').textContent= totalSubs;

}

function runSubs() {
var subsNo = Number(document.getElementById('subsNo').textContent);
var NewSubsNo = 0;
NewSubsNo = subsNo + 1;
document.getElementById('subsNo').textContent= NewSubsNo;
return NewSubsNo; 
}

totalSubs = runSubs(totalSubs);

<input type="submit" value="Subscribe" style="width:200px; height:60px; font-size: 2.0rem;" onclick="runSubs()"  onload="getSubs()" />

<p id="subsNo"> </p>

Backend:

var http = require('http');
var fs = require('fs'); 
http.createServer(function (req, res) {   
  fs.readFile('MobileUI.html', function(err, data) {     
    res.writeHead(200, {'Content-Type': 'text/html'});
  res.write(data);
  return res.end();
  });
}).listen(8080);