node.js

//    ____  _            _     _____           _       
//   |  _ \| |          | |   / ____|         | |      
//   | |_) | | __ _  ___| | _| (___  _   _  __| | ___  
//   |  _ <| |/ _` |/ __| |/ /\___ \| | | |/ _` |/ _ \ 
//   | |_) | | (_| | (__|   < ____) | |_| | (_| | (_) |
//   |____/|_|\__,_|\___|_|\_|_____/ \__,_|\__,_|\___/ Sudan Dev -->
	   
//			 # by Omer A. Bila 
//			 # FB : https://web.facebook.com/BlackSudo :D

var keys = document.querySelectorAll('#calculator span');
var operators = ['+', '-', '*', '/'];
var decimalAdded = false;

for(var i = 0; i < keys.length; i++) {
	keys[i].onclick = function(e) {
		
		var input = document.querySelector('.screen');
		var inputVal = input.innerHTML;
		var btnVal = this.innerHTML;
		
		if(btnVal == 'AC') {
			input.innerHTML = '';
			decimalAdded = false;
		}
		
		else if(btnVal == '=') {
			var equation = inputVal;
			var lastChar = equation[equation.length - 1];
			
			
			equation = equation.replace(/x/g, '*').replace(/÷/g, '/');
			
			
			if(operators.indexOf(lastChar) > -1 || lastChar == '.')
				equation = equation.replace(/.$/, '');
			
			if(equation)
				input.innerHTML = eval(equation);
				
			decimalAdded = false;
		}
		
		else if(operators.indexOf(btnVal) > -1) {
			
			var lastChar = inputVal[inputVal.length - 1];
			
			if(inputVal != '' && operators.indexOf(lastChar) == -1) 
				input.innerHTML += btnVal;
			
			else if(inputVal == '' && btnVal == '-') 
				input.innerHTML += btnVal;
			
			if(operators.indexOf(lastChar) > -1 && inputVal.length > 1) {

				input.innerHTML = inputVal.replace(/.$/, btnVal);
			}
			
			decimalAdded =false;
		}
		
		else if(btnVal == '.') {
			if(!decimalAdded) {
				input.innerHTML += btnVal;
				decimalAdded = true;
			}
		}
		else {
			input.innerHTML += btnVal;
		}
		
		e.preventDefault();
	} 
}