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HTML

              
                <!-- HTML & CSS for this project is made by https://codepen.io/merkund/pen/rQjKGj?page=4, all credits due  -->  


<div class="container">
    <p>Too lazy to calculate, eh?*&nbsp;🙈</p>
  <div class="calc">
    <div class="calc-display">0</div>
    <!--Keys-START-->
    <div class="calc-key">
      <button class="key-op" data-action="add">+</button>
      <button class="key-op" data-action="substract">-</button>
      <button class="key-op" data-action="multiply">&times;</button>
      <button class="key-op" data-action="divide">÷</button>
      <button class="numpad">7</button>
      <button class="numpad">8</button>
      <button class="numpad">9</button>
      <button class="numpad">4</button>
      <button class="numpad">5</button>
      <button class="numpad">6</button>
      <button class="numpad">1</button>
      <button class="numpad">2</button>
      <button class="numpad">3</button>
      <button class="numpad">0</button>
      <button class="numpad">.</button>
      <button data-action="clear">AC</button>
      <button class="key-eq" data-action="equate">=</button>
    </div>
    </div>
    <!-- <div class="themer">
    <button class="switch opt1"></button>
      <button class="switch opt2"></button>
      <button class="switch opt3"></button>
    </div>
    <p class="small-text">Try changing colours, too!*</p> -->
    </div>
    </div>
              
            
!

CSS

              
                /*  HTML & CSS for this project is made by https://codepen.io/merkund/pen/rQjKGj?page=4, all credits due */

@import url("https://fonts.googleapis.com/css?family=Quicksand:400,500");
html {
  box-sizing: border-box;
}

*,
*::before,
*::after {
  box-sizing: inherit;
}

body {
  align-items: center;
  display: flex;
  height: 100vh;
  justify-content: center;
}

body,
p {
  margin: 0;
}

html {
  display: grid;
  min-height: 100%;
}

body {
  display: grid;
  font-family: "Quicksand", sans-serif;
  background: #8EC5FC linear-gradient(62deg, #8EC5FC 0%, #E0C3FC 100%);
}

.container {
  max-width: 20em;
}

.container > p {
  text-align: center;
}

p {
  color: #333;
  font-size: 2em;
}

.small-text {
  margin-top: 1em;
  font-size: 1.5em;
}

.mini-text {
  font-size: 0.9em;
  margin-top: 1em;
}

.calc {
  background: #ffffff;
  margin: auto;
  position: relative;
  height: 24em;
  width: 20em;
  margin-top: 2em;
  margin-bottom: 2em;
  overflow: hidden;
  border-radius: 20px;
  box-shadow: 0 0 40px 0px rgba(0, 0, 0, 0.3);
}

.calc-display {
  background: #ffffff;
  color: #000;
  font-size: 2.5em;
  padding: 0.5em 0.75em;
  text-align: right;
}

.calc-key {
  display: grid;
  grid-gap: 0px;
  grid-template-columns: repeat(4, 1fr);
}

.calc-key > * {
  padding: 1.25em 1.25em;
  position: relative;
  text-align: center;
}

.calc-key > *:active::before,
.calc-key > .is-depressed::before {
  background: #8EC5FC;
  content: "";
  bottom: 0;
  left: 0;
  right: 0;
  top: 0;
  opacity: 0.3;
  position: absolute;
  z-index: 1;
}

button {
  border: 0;
  border-radius: 0;
  background: transparent;
  font-size: inherit;
  font-family: inherit;
  font-weight: inherit;
  outline: none;
  appearance: none;
  text-align: left;
}

button:hover,
button:active,
button:focus {
  outline: none;
}

.key-op {
  background: #eee;
  font-size: 1.25em;
  padding: 0.8em;
}

.key-eq {
  background: #333;
  color: #ffffff;
  grid-column: -2;
  grid-row: 2/span 4;
  font-size: 1.25em;
}

.themer {
  margin: auto;
  display: flex;
  justify-content: space-between;
  max-width: 10em;
}

.switch {
  padding: 1.1em;
  border-radius: 50%;
}

.opt1 {
  background: #2196f3;
}

.opt2 {
  background: #9c27b0;
}

.opt3 {
  background: #e91e63;
}

              
            
!

JS

              
                // HTML & CSS for this project is made by https://codepen.io/merkund/pen/rQjKGj?page=4, all credits due

// Feture Wishlist
// Display used operator along with number in UI
// -/+ Operator in Calculator


class Calc {
    static reset() {
        Calc.sum = undefined;
        Calc.operatorMemory = undefined;
        Calc.equated = undefined;
    }
    static add(a, b) {
        return a + b;
    }
    static substract(a, b) {
        return a - b;
    }
    static multiply(a, b) {
        return a * b;
    }
    static divide(a, b) {
        return a / b;
    }

    static operate(input, operator) {

        //debugger;

        if (Calc.operatorMemory === undefined) {
            Calc.operatorMemory = operator;
        }

        if (Calc.sum === undefined) {
            Calc.sum = input;

        }

        // Addresses User Continuing Calculation after equating, ie for input 2+5=(7)*2= can return 14.
        else if (Calc.equated && Number.isNaN(input)) {
                Calc.equated = false;
        }
        // Addresses User Switching Operator in Between Calculation, ie for input 2+*5 will return 2*5;
        else if(Number.isNaN(input) && Calc.sum != undefined){
            Calc.operatorMemory = operator;
            return;
        }
        else {

            // Run Calculation
            Calc.sum = Calc[Calc.operatorMemory](Calc.sum, input);

            // Check if Operator is not equate
            if (!(operator === 'equate')) {
                // Updating Operator Memory to Newly Selected Operator for Next Calculation
                Calc.operatorMemory = operator;
                Calc.equated = false;
            } else {
                // Reset Operator Memory 
                Calc.operatorMemory = undefined;
                Calc.equated = true;
            }

        }
        return (Calc.sum);

    }
}
// Declaring Calc Variables
Calc.sum;
Calc.operatorMemory;
Calc.equated;

class Event {
    static captureInput() {
        Event.inputSource.addEventListener('click', (e) => {


            // Check if User Clicked on a Number and then Capture Number as string
            if (e.target.className === 'numpad') {
                
                // Check if Last Input was Equate, If so Reset Calc for Fresh Calculation
                if(Event.inputMemory === 'equate'){
                    Event.reset();
                    Calc.reset();
                    UI.display(0);
                }

                // Adding on to Event.inputString the input values

                // This code is to prevent multiple decimal points in input
                // Check if Input is Decimal
                if (/\./.test(e.target.textContent)) {
                    // Check if Event.input already has a Decimal to Avoid Multiple Decimal Points
                    if (!(/\./.test(Event.inputString))) {
                        // Check if user input is within UI Display Limit
                        if(Event.inputString.length < UI.displayLimit){Event.inputString += e.target.textContent}
                    }
                } else /*If Input is a number, skips the decimal logic*/ {
                    // Check if user input is within UI Display Limit
                    if(Event.inputString.length < UI.displayLimit){Event.inputString += e.target.textContent}
                }

                // Display Input on UI
                UI.display(Event.inputString);

                // Since Clicked Event is Numpad, Sets lnput Memory as Number. See Line 107 and 119 for Clarity
                Event.inputMemory = 'number';

            }
            // Check if User Clicked on an Operator and Run Operation
            else if (e.target.className === 'key-op' || e.target.className === 'key-eq') {

                // To Prevent Same Operator being Clicked Multiple times, see line 119 and 107 for clarity
                if(!(Event.inputMemory === e.target.dataset.action)){
                    Event.selectedOperator = e.target.dataset.action;

                    // Convert Input String to Floating Point Number, Run Calculation & Display in UI 
                    console.log(Event.inputString,Event.selectedOperator);
                    UI.display(Calc.operate(parseFloat(Event.inputString), Event.selectedOperator));
    
                    // Clear Input String
                    Event.inputString = '';
    
                    // Saves Clicked Operator to Memory
                    Event.inputMemory = e.target.dataset.action;
                } 

            }
            // If User Clicked On Clear Reset Calc
            else if (e.target.dataset.action === 'clear') {
                Event.reset();
                Calc.reset();
                UI.display(0);
            }
        });
    }

    static reset(){
        Event.inputString = '';
        Event.selectedOperator = undefined;
        Event.inputMemory = '';
    }

    static init() {
        document.addEventListener("DOMContentLoaded", function () {
            console.log('Dom Loaded');
            Event.captureInput();
        });
    }
}
// Declaring Event Variables
Event.inputSource = document.querySelector('.calc-key');
Event.inputString = '';
Event.selectedOperator;
Event.inputMemory = '';

class UI {
    static display(value) {
        if(typeof(value)==="string"){
            // Limit String to Display Limit of Calculator
            UI.displayBar.textContent = value.substring(0, UI.displayLimit);
        }
        else if(typeof(value)==="number"){
            // Check if Computed Number is within UI display Limit
            if(value < ((Math.pow(10,UI.displayLimit))-1)){
    
                // Compute Trailing Decimals That can be shown without UI Overflow
                UI.displayableDecimalTrail =  (UI.displayLimit) - (value.toString().split(".")[0].length);
                // Limit Trailing Decimal's Digits
                if(value %1 != 0){
                    UI.displayBar.textContent = parseFloat(value.toFixed(UI.displayableDecimalTrail)); // parseFloat used to remove insignificant trailing zeros
                } else {
                    UI.displayBar.textContent = value;
                } 
        
                // If number is more than UI display Limit, Show Error
                } else {
                UI.displayBar.textContent = 'E';
                }
        }
    }
}
// Declaring UI Variables
UI.displayBar = document.querySelector('.calc-display');
UI.displayLimit = 12;
UI.displayableDecimalTrail;



// Listen For Input
Event.init(); 
              
            
!
999px

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