Prevent second click with ( preventDefault )

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<html lang="en">
<head>
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    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Prevent second click with ( preventDefault )</title>
  <style>


    body {
      cursor: context-menu;
      margin: 0;
      font-family: calibri;
      overflow-x: hidden;
    }

    button {
      border: none;
      outline: none;
      background-color: red;
      color: white;
    }

    textarea {
      border: none;
      background-color: white;
      resize: none;
      outline: none;
    }

    a {
      text-decoration: none;
    }

    audio {
      background-color: rgba(128, 128, 128, 0.1);
    }

             /*  ------------  default  ---------------   */


    #txt {
      font-size: 5vw;text-align:center;
    }


    #btn {
      margin: 10px;
      padding: 5px 20px;
    }


    @media all and (orientation:portrait) and (max-width:425px) {
      #txt {
        font-size: 15vw;
      }
    }

    @media all and (orientation:landscape) and (max-width:538px) {
      #txt {
        font-size: 10vw;
      }
    }

    @media all and (orientation:landscape) and (min-width:539px) and (max-width:768px) {
      #txt {
        font-size: 8vw;
      }
    }
  </style>
</head>

<body>

  <center><button id='btn'>Prevent second click with ( preventDefault )</button></center>
  <h1 id='txt'></h1>


  <script>
    i = 0;
    def = true;

    btn.addEventListener('click', function() {
      if (def) {
        foo();
        def = false;
      } else {
        event.preventDefault();
      }
    });

    (foo = () => {
      setInterval(() => {
        txt.innerHTML = i;
        i++;
      }, 500);
    })

  </script>
</body>
</html>