Javascript: url containing random number

I would add a parameter but you can leave it out if needed:

var url = "http://www.mypage.com/index.php?rnd="+Math.random()

or

var url = "http://www.mypage.com/index.php?rnd="+new Date().getTime()

Link:

<a href="http://www.mypage.com/index.php?rnd=1" onClick="this.href=this.href.split('?')[0]+'?rnd='+new Date().getTime()">Mostly random</a>

Note that if you have more than one assignment - for example in a loop, you need to add to the getTime since an iteration of the loop is faster than a millisecond:

var rnd = new Date().getTime();
for (var i=0;i<links.length;i++) {
   links[i].href = "http://www.mypage.com/index.php?rnd="+(rnd+i);
}

UPDATE to use the URL constructor with searchParams

const addRnd = urls => {
  let rnd = new Date().getTime();
  return urls.map((urlStr,i) => {
    let url = new URL(urlStr);
    url.searchParams.set("rnd",rnd+i);  // in case called multiple times
    return url;
  });
};
const urls = addRnd( ["http://www.mypage.com/index1.php","http://www.mypage.com/index2.php","http://www.mypage.com/index3.php"])
console.log(urls)

var lower = 0;
var upper = 100000000;
var url = "http://www.mypage.com/index.php?"+(Math.floor(Math.random()*(upper-lower))+lower)

it generates a random X from 0(lower) to 100000000 (upper), you can obv set the bounds you want ;)

<a href="http://www.mypage.com/index.php?" onclick="this.href+=new Date().getTime();return true;">link</a>