JSFiddle - React, Tailwind, and code Playground

JavaScript

var wins = 0;
var iterations = 10000000;

for(i=0;i<iterations;i++) {
  var prior = Math.random();
  var hand1 = Math.random();
  var hand2 = Math.random();

  // Avoid the case where we get non distinct numbers
  if(hand1 == hand2) { continue; }

  if(hand1 < prior) {
    if(hand2 > hand1) { wins += 1; }
  } else {
    if(hand2 < hand1) { wins += 1; }
  }
}

document.body.innerHTML = "Win probability: " + wins/iterations;