JSFiddle - React, Tailwind, and code Playground

HTML

<script>
	var grades = [
	  'Хуйня', 'Комон', 'Зеленый', 'Рар', 'Фиол', 'Эпик', 'Легенда', 'Релик', 'Э. Чудес', 'Э. Сказаний', 'Э. Легенд', 'Э. Мифов'
	];

	var success_chances = [
	  10000, 10000, 4000, 3000, 3000, 3000, 2500, 1000, 750, 500, 250, 0
	];

	var great_chances = [
	  2000, 2000, 2000, 2000, 2000, 2000, 2000, 2000, 2000, 2000, 0, 0
	];

	var break_chances = [
	  0, 0, 0, 0, 0, 0, 0, 5000, 10000, 10000, 10000, 0
	];

	var downgrade_chances = [
	  0, 0, 0, 0, 0, 4000, 4000, 10000, 0, 0, 0, 0
	];

	var downgrade_mins = [-1, -1, -1, -1, -1, 4, 4, 4, 5, 6, 7, -1];

	var catalyst_chances = [
	  1, 1.5, 1.5, 1.5, 1.75, 1.75, 2, 2.5, 2.5, 2.5, 2.5, 0
	];

	var tries = 0;
	var breaks = 0;
	var gsuccess = 0;

	function getGrade() {
	  return document.getElementById("starting_grade_selector").value;
	}

	function findGradeId(selected_grade) {
	  for (var i = 0; i < grades.length; ++i) {
	    if (grades[i] == selected_grade) {
	      return i;
	    }
	  }
	}

	function calculate(sourceGrade) {
	  var selected_grade = getGrade();
	  var target_grade = document.getElementById("target_grade_selector").value;

	  if (selected_grade == target_grade) {
	    return;
	  }

	  tries++;

	  var grade = findGradeId(selected_grade);

	  var success_chance = success_chances[grade] / 100;
	  var great_success_chance = great_chances[grade] / 100;
	  var break_chance = break_chances[grade] / 100;
	  var downgrade_chance = downgrade_chances[grade] / 100;
	  var downgrade_min = downgrade_mins[grade];
	  var catalyst = document.getElementById("catalyst").checked;
	  var strongscroll = document.getElementById("use_strong").checked;
	  var strong_leg_only = document.getElementById("only_leg").checked;

	  if (catalyst) {
	    success_chance *= catalyst_chances[grade];
	  }

	  var roll = rng();

	  if (roll <= success_chance) {
	    roll_x2_scroll = rng();

	    if (((strongscroll && strong_leg_only && (grade == 6)) || (strongscroll && !strong_leg_only)) && (roll_x2_scroll <=...

JavaScript

grades.forEach(function(grade) {
	  var option = document.createElement("option");
	  option.text = grade;
	  document.getElementById("starting_grade_selector").add(option);
	  option = document.createElement("option");
	  option.text = grade;
	  document.getElementById("target_grade_selector").add(option);
	  setGrade(2);
	  document.getElementById("target_grade_selector").value = grades[8];
	});