document.addEventListener('keydown', function(event) {
	if (event.keyCode == 8) {
		let oldstr = document.getElementById("display").value
		let newstr = oldstr.substring(0, oldstr.length - 1);
		document.getElementById("display").value = newstr;
	}
})

function ce() {
	let oldstr = document.getElementById("display").value
	let newstr = oldstr.substring(0, oldstr.length - 1);
	document.getElementById("display").value = newstr;
}

function clearall() {
	document.getElementById("display").value = "0";
	document.getElementById("expression").value = "";
}

function operator(sign) {
	window.entered = "false";
	let displayed_value = document.getElementById("display").value;
	let existing_operator = displayed_value.charAt(displayed_value.length-1);
	if (displayed_value === '0') {
		document.getElementById("display").value = sign;
	} else if (displayed_value === "Error") {
		document.getElementById("display").value = window.previous_expression;
	} else if (existing_operator == '/' || existing_operator =='*' || existing_operator =='+' || existing_operator == '-' || existing_operator == '.') {
		let oldstr = displayed_value.substring(0, displayed_value.length - 1);
		document.getElementById("display").value = oldstr + sign;
	}
	else {

		document.getElementById("display").value += sign;
	}
}

function equals() {
	window.entered = "false";
	try {
		let expression = document.getElementById("display").value;
		window.previous_expression = expression;
		if (expression === "Error") {
			document.getElementById("expression").value = previous_expression;
			expression.value = "Error";

		} else {
			window.entered = "true";
			document.getElementById("expression").value = previous_expression;
			document.getElementById("display").value = eval(document.getElementById("display").value);
		}
	} catch (e) {
		document.getElementById("expression").value = previous_expression;

		document.getElementById("display").value = "Error";
	}
}

function number(obj) {

	let displayed_value = document.getElementById("display").value;

	if (displayed_value === '0') {
		document.getElementById("display").value = obj;
	} else if (displayed_value === "Error")
	{
		window.entered = "false";
		document.getElementById("display").value = window.saved_expression + obj;
	} else {
		if (window.entered == "true") {
			document.getElementById("display").value = obj;
			window.entered = "false";
		} else {
			document.getElementById("display").value += obj;
			window.entered = "false";
		}

	}
}