Creating a China friendly captcha w/ Jquery & Rails view

chinaFriendlyCaptcha.js

function getCnNumber(x){
  if (x > 10 && x <= 20){
    const ch = '零一二三四五六七八九十'.split('').map((c, i) => x === i && c).find(s => s);
    return '十' + ch;    
  }
  else
    return '零一二三四五六七八九十'.split('').map((c, i) => x === i && c).find(s => s);
}

function solveHuman(ans){
  const s = '<input type="submit" value="Submit Registration" class="btn requisLime fs-24" style="height:60px;width:300px;"/>';
  
  const humanSolution = $('#human-solution').val();

  const solution = parseInt(ans);
  const cnSolution = getCnNumber(ans);

  if (parseInt(humanSolution) === solution || humanSolution === cnSolution){
    $('#q-block').remove();
    $("#prob-canvas").remove();
    $("#asimov-gate").html(s);
  } 
}


function asimovGate(){
  $('[data-js-prove-humanity]').click(function(event){
    
    $("#prove-it").hide();

    $("#prob-canvas").show();
    const data = $(this).attr('button-func');
    $('#prompt').remove();
    eval(data);

  $('#solve-button').click(function(){
    const data = $(this).attr('button-func');
    eval(data);
  });

  const a = Math.floor(Math.random() * Math.floor(10));
  const b = Math.floor(Math.random() * Math.floor(10));
  // chinese simplified numbers
  const c = getCnNumber(a);
  const d = getCnNumber(b);

  const answer = `${a + b}`;

  const verify = `<div id="q-block"><div id="human-question"></div><input id="human-solution"></input><br/><button type="button" class="btn requisLime" style="margin-top:12px;padding:0 15px"id="solve-button" onclick="solveHuman(${answer})">Solve - 具求解此方程</button></div>`
  let canvas = document.getElementById('prob-canvas');
  let canvasText = canvas.getContext("2d");
  const textString = `${a} + ${b}\n${c} 加 ${d}`;
  let lines = textString.split('\n');
  canvasText.font = "36px san-serif";
  
  for (let i = 0; i < lines.length; i++)
    canvasText.fillText(lines[i], 50, 50 + (i*31) );
  
  $('#asimov-gate').append(verify);
  $('#prove-it').remove();
  event.preventDefault;
  })
}

view.html.erb

<%= form_with model: @user, local: true do |f|%>
  <%# other form elements here %>
  <canvas id="prob-canvas" width="200" height="100" style="border:1px solid #d3d3d3;display:none;">
    Your browser does not support the canvas element.
  </canvas>
  <div class="actions py-4" id="asimov-gate"/>
<% end %>