<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8"/>
    <title>Calculator Demo</title>
    
</head>
<body onload="load()">
    <div id="calculator">
        
        <div class="top">
            <span id="clear">C</span>
            <div id="screen"></div>
        </div>

*{
    margin: 0;
    padding: 0;
    /* Padding and border are included in the element's total width and height */
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;

font:bold 14px Aria,sans-serif;
}

html {
    height: 100%;
    background: #111;
    background-size: cover;
}

#calculator {
    width: 325px;
    height: auto;

margin: 100px auto;
    padding: 20px 20px 9PX;

background: #0b8fcc;
    background: linear-gradient(#0757ce, #8bceec);
    border-radius: 3px;
    /* Using box shadows to create 3D effects */
    box-shadow: 0px 4px #009de4, 0px 10px 15px rgba(0,0,0,0.2);
}

#screen {
    float: right;

height: 40px;
    width: 212px;

padding: 0 10px;

background: rgba(0, 0, 0, 0.2);
    border-radius: 3px;
    /* Inset shadow on the screen to create indent */
    box-shadow: inset 0px 4px rgba(0, 0, 0, 0.2);

/* Typography */
    font-size: 17px;
    line-height: 40px;
    color: white;
    text-shadow: 1px 1px 2px rgba(0, 0, 0, 0.2);
    text-align: right;
    letter-spacing: 1px;
}

/* Clear Floats to make sure #calculator includes them correctly */
.keys, .top {
    overflow: hidden;
}

/* Applying css on all the keys */
.keys span, #clear {
    float: left;
    position: relative;
    top: 0;
    /* Specifies the type of cursor to be displayed when pointing on an element. */
    cursor: pointer;

width: 66px;
    height: 36px;

background: white;
    border-radius: 3px;
    box-shadow: 0px 4px rgba(0, 0, 0, 0.2);

margin: 0 7px 11px 0;

color: #888;
    line-height: 36px;
    text-align: center;

/* prevent selection of text inside keys */
    user-select: none;

/* Transitions allows you to change property values smoothly  */
    transition: all 0.2s ease;
}

.keys span.operator {
    background: #FFF0F5;
    margin-right: 0; /* 7px in previous setting*/
}

/* Eval Button */
.keys span.eval {
    background: #f1ff92;
    box-shadow: 0px 4px #9da853;
    color: #888e5f;
}

.keys span.eval:hover {
    background: #abb850;
    box-shadow: 0px 4px #717a33;
    color: #ffffff;
}

.keys span.eval:active {
    box-shadow: 0px 0px #717a33;
    top: 4px;
}

/* Clear Button */
#clear {
    background: #ff9fa8;
    box-shadow: 0px 4px #ff7c87;
    color: white;
}

#clear:hover {
    background: #f68991;
    box-shadow: 0px 4px #d3545d;
    color: white;
}

#clear:active {
    top: 4px;
    box-shadow: 0px 0px #d3545d;
}

/* Other Buttons */
.keys span:hover {
    background: #9c89f6;
    box-shadow: 0px 4px #6b54d3;
    color: white;
}

.keys span:active {
    box-shadow: 0px 0px #6b54d3;
    top: 4px;
}

/**
 * Created by chuandong on 15/11/27.
 */

function load() {
  var btns = document.querySelectorAll("#calculator span");
  var operators = ["+", "-", "x", "÷"];
  var inputScreen = document.querySelector("#screen");
  var btnValue;
  var input;

for (var i = 0; i < btns.length; i++) {
    var decimalAdded = false; // Flag used to avoid two decimal

btns[i].addEventListener("click", function() {
      btnValue = this.innerHTML;
      input = inputScreen.innerHTML;

switch (btnValue) {
        case "C":
          inputScreen.innerHTML = "";
          decimalAdded = false;
          break;
        case "=":
          // Last char of string
          var lastChar = input[input.length - 1];

// Replace x to *, + to / which could be calculated in eval
          input = input.replace(/x/g, "*").replace(/÷/g, "/");

// Checking the last character of the input.
          // If it's an operator or a decimal, remove it
          // /.$/ means last char in regex
          if (operators.indexOf(lastChar) > -1 || lastChar == ".")
            input = input.replace(/.$/, "");

if (input) {
            // If the argument is an expression, eval() evaluates the expression.
            // If the argument is one or more JavaScript statements, eval() executes the statements.
            inputScreen.innerHTML = eval(input);
          }
          decimalAdded = false;
          break;
        case ".":
          if (!decimalAdded) {
            inputScreen.innerHTML += btnValue;
            decimalAdded = true;
          }
          break;
        case "+":
        case "-":
        case "x":
        case "÷":
          // Last char of string
          var lastChar = input[input.length - 1];

// Only add operator if input is not empty and there is no operator at the last
          if (input != "" && operators.indexOf(lastChar) == -1)
            inputScreen.innerHTML += btnValue;
          // Allows minus if the string is empty. The first number could be under zero
          else if (input == "" && btnValue == "-")
            inputScreen.innerHTML += btnValue;

// Allows to represent the last operation
          if (operators.indexOf(lastChar) > -1 && input.length > 1) {
            inputScreen.innerHTML = input.replace(/.$/, btnValue);
          }
          decimalAdded = false;
          break;
        default:
          inputScreen.innerHTML += btnValue;
          decimalAdded = false;
          break;
      }
    });
  }
}

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