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JavaScript preprocessors can help make authoring JavaScript easier and more convenient. For instance, CoffeeScript can help prevent easy-to-make mistakes and offer a cleaner syntax and Babel can bring ECMAScript 6 features to browsers that only support ECMAScript 5.

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HTML

            
                <div class="container-fluid"><!--calculator body-->
    <div class="row">
      <div class="col-xs-12">
        <input id="screen" type="text" disabled>
      </div>
    </div>
    <div class="row">
      <div class="col-xs-8"><u>freeCodeCamp</u></div>
      <div class="col-xs-4"><input class="btn btn-default m-op-btn" type="button" value="on/c"  onclick="reset()"></div>
    </div>
    <div class="row">
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="7" onclick="keyEntry7()"></div>
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="8" onclick="keyEntry8()"></div>
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="9" onclick="keyEntry9()"></div>
      <div class="col-xs-3"><input class="btn btn-default op-btn" type="button" value="/" onclick="division()"></div>
    </div>
    <div class="row">
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="4" onclick="keyEntry4()"></div>
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="5" onclick="keyEntry5()"></div>
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="6" onclick="keyEntry6()"></div>
      <div class="col-xs-3"><input class="btn btn-default op-btn" type="button" value="X" onclick="multiplication()"></div>
    </div>
    <div class="row">
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="1" onclick="keyEntry1()"></div>
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="2" onclick="keyEntry2()"></div>
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="3" onclick="keyEntry3()"></div>
      <div class="col-xs-3"><input class="btn btn-default op-btn" type="button" value="-" onclick="subtraction()"></div>
    </div>
    <div class="row">
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="0" onclick="keyEntry0()"></div>
      <div class="col-xs-3"><input class="btn btn-default" type="button" value="." onclick="decimal()"></div>
      <div class="col-xs-3"><input class="btn btn-default m-op-btn" type="button" value="=" onclick="equals()"></div>
      <div class="col-xs-3"><input class="btn btn-default op-btn" type="button" value="+" onclick="addition()"></div>
    </div>
  </div>
!

CSS

            
              body, html {
 background-color: #666;
}

u {
  font-family: "Ubuntu Mono";
  font-size: 1.5em;
}

.btn {
  width: 60px;
  box-shadow: 1px 1px 5px #d2d2d2;
  border-radius: 9px;
  background-color: #e1ffef;
  font-weight: bold;
  font-size: 1.4em;
  padding: 10px;
  text-align: center;
}

.btn:hover {
  background-color: #D7F5E5;
}

.op-btn {
  width: 45px;
  background-color: #BA1E38;
  color: white;
}

.op-btn:hover {
  background-color: #851729;
  color: white;
}

.m-op-btn {
  background-color: #5f9aa4;
  color: white;
  width: 75px;
  margin-left: -7px;
}

.m-op-btn:hover {
  background-color: #578d96;
  color: white;
}

.container-fluid {
  margin-top: 20px;
  width: 300px;
  height: auto;
  background-color: #f2f2f2;
  box-shadow: inset 0 4px 8px 0 rgba(0, 0, 0, 0.2), inset 0 6px 20px 0 rgba(0, 0, 0, 0.19);
  padding: 15px;
  border-radius: 25px;
}

.row {
  padding-top: 5px;
  padding-bottom:5px;
}

#screen {
  background-color: grey;
  font-family: "Inconsolata";
  font-size: 2em;  
  width: 100%;
  margin: auto;
  height: 60px;
  line-height: 60px;
  text-align: right;
  font-weight: bold;
  padding: 10px;
  box-shadow: inset 0 4px 8px 0 rgba(0, 0, 0, 0.2), inset 0 6px 20px 0 rgba(0, 0, 0, 0.19);
}
!

JS

            
              var screenDisplay = document.getElementById("screen");
var current = "";
var equation;
var a = "";
var b = 0;
var c;

function update() {
  screenDisplay.value = current; 
  document.getElementById("screen").style.backgroundColor = "#8EFC5B"; 
}

function reset() {
  current = "";
  a = 0;
  b = 0;
  c = 0;
  update();
}

function keyEntry0() {
   current += "0";
   update();
}

function keyEntry1() {
   current += "1";
   update();
}

function keyEntry2() {
   current += "2";
   update();
}

function keyEntry3() {
   current += "3";
   update();
}

function keyEntry4() {
   current += "4";
   update();
}

function keyEntry5() {
   current += "5";
   update();
}

function keyEntry6() {
   current += "6";
   update();
}

function keyEntry7() {
   current += "7";
   update();
}

function keyEntry8() {
   current += "8";
   update();
}

function keyEntry9() {
   current += "9";
   update();
}

function decimal() {
  current += ".";
  update();
}

function division() {
  a += current + "/";
  update();
  current = "";
}

function multiplication() {
  a += current + "*";
  update();
  current = "";
}

function subtraction() {
  a += current + "-";
  update();
  current = "";
}

function addition() {
  a += current + "+";
  update();
  current = "";
}

function equals() {
  current = a + current;
  current = eval(current);
  update();
  a = "";
}
!
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