<h1>Function</h1>
    <h2>Basic</h2>
    <ul>
      <script>
        function two(){
          document.write('<li>2-1</li>');
          document.write('<li>2-2</li>');
        }
        document.write('<li>1</li>');
        two();
        document.write('<li>3</li>');
        two();
      </script>
    </ul>
    <h2>Parameter & Argument</h2>
    <script>
      function onePlusOne(){
        document.write(1+1+'<br>');
      }
      onePlusOne();
      function sum(left, right){
        document.write(left+right+'<br>');
      }
      function sumColorRed(left, right){
        document.write('<div style="color:red">'+left+right+'</div><br>');
      }
      sum(2,3); // 5
      sumColorRed(2,3); // 5
      sum(3,4); // 7
    </script>
    <h2>Return</h2>
    <script>
      function sum2(left, right){
        return left+right;
      }
      document.write(sum2(2,3)+'<br>');
      document.write('<div style="color:red">'+sum2(2,3)+'</div>');
      document.write('<div style="font-size:3rem;">'+sum2(2,3)+'</div>');
    </script>

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