index.html

<link rel="stylesheet" href="style.css">
<center>
  <h3>Калькулятор</h3>
  <h2>Простой, обычный калькулятор.</h2>
  <script>
    function clean() {
      var output = document.getElementById("output");
      output.value = "";
    }
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      var output = document.getElementById("output");
      output.value += "1";
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      output.value += "2";
    }
    function three() {
      var output = document.getElementById("output");
      output.value += "3";
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    function four() {
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      output.value += "4";
    }
    function five() {
      var output = document.getElementById("output");
      output.value += "5";
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    function six() {
      var output = document.getElementById("output");
      output.value += "6";
    }
    function seven() {
      var output = document.getElementById("output");
      output.value += "7";
    }
    function eight() {
      var output = document.getElementById("output");
      output.value += "8";
    }
    function nine() {
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      output.value += "9";
    }
    function zero() {
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      output.value += "0";
    }
    function divide() {
      var output = document.getElementById("output");
      output.value += "/";
    }
    function multiply() {
      var output = document.getElementById("output");
      output.value += "*";
    }
    function plus() {
      var output = document.getElementById("output");
      output.value += "+";
    }
    function minus() {
      var output = document.getElementById("output");
      output.value += "-";
    }
    function stepen() {
      var output = document.getElementById("output");
      output.value += "**";
    }
    function tochka() {
      var output = document.getElementById("output");
      output.value += ".";
    }
    function calculate() {
      var output = document.getElementById("output");
      output.value = eval(output.value);
    }
  </script>
  <input id="output" readonly>
  <div id="row">
    <button onclick="one()">1</button>
    <button onclick="two()">2</button>
    <button onclick="three()">3</button>
    <br>
  </div>
  <div id="row">
    <button onclick="four()">4</button>
    <button onclick="five()">5</button>
    <button onclick="six()">6</button>
    <br>
  </div>
  <div id="row">
    <button onclick="seven()">7</button>
    <button onclick="eight()">8</button>
    <button onclick="nine()">9</button>
    <br>
  </div>
  <div id="row">
    <button onclick="divide()">/</button>
    <button onclick="zero()">0</button>
    <button onclick="multiply()">x</button>
    <br>
  </div>
  <div id="row">
    <button onclick="minus()">-</button>
    <button onclick="plus()">+</button>
    <button onclick="clean()">CE</button>
    <br>
  </div>
  <div id="row">
    <button onclick="stepen()">x²</button>
    <button onclick="calculate()">=</button>
    <button onclick="tochka()">.</button>
  </div>
</center>
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