script.js

var memory = "0",
	currentValue = "0",
	resultValue = 0;

const maxChar = 10;

const display = document.querySelector(".display p");

//printing a value on the display

function addValue(digit) {
	if (eval(currentValue) === 0 && currentValue.indexOf(".") === -1) {
		currentValue = digit;
	} else {
		currentValue += digit;
	}

	display.innerHTML = currentValue;
}

//adding a decimal

function addDecimal() {
	if (currentValue.length === 0) {
		currentValue = "0.";
	} else if (currentValue.indexOf(".") === -1) {
		currentValue += ".";
	}
	display.innerHTML = currentValue;
}

//inclunding the minusplus function

function minusPlus() {
	if (currentValue.indexOf("-") !== 0)
		return (currentValue = "-" + currentValue);

	if (currentValue.indexOf("-") === 0)
		return (currentValue = currentValue.substring(1));

	if (parseFloat(currentValue) === 0 && currentValue.indexOf(".") === -1)
		return (currentValue = 0);

	// if (currentValue.indexOf("-") === 0) {
	// 	currentValue = currentValue.substring(1);
	// } else {
	// 	currentValue = "-" + currentValue;
	// }
	// if (eval(currentValue) === 0 && currentValue.indexOf(".") === -1) {
	// 	currentValue = 0;
	// }
}

// adding the clear function to C

function clearAll() {
	currentValue = "0";
	resultValue = 0;
	memory = "0";

	display.innerHTML = currentValue;
}

// adding the operators

function addOperation(op) {
	if (resultValue !== 0) {
		//if the user inputs a string of values and operations
		calculate(); //the calculator only deals with two calculations
	}

	if (op.indexOf("*") > -1) {
		//instead of using html.value,
		resultValue = 1; // these are the operations
	} //
	if (op.indexOf("/") > -1) {
		resultValue = 2;
	}
	if (op.indexOf("+") > -1) {
		resultValue = 3;
	}
	if (op.indexOf("-") > -1) {
		resultValue = 4;
	}

	memory = currentValue; // storing every entry in the memory variable to always calculate 'current value' against
	currentValue = ""; // clear the current value so it can be used next, after being stored in the memory

	display.innerHTML = currentValue;
}

// calculating percentages

function percentage() {
	if (eval(memory) === 0) {
		currentValue = currentValue / 100;
	} else {
		currentValue = (currentValue / 100) * memory;
	}

	display.innerHTML = currentValue;
}

// adding the four core calculation possibilities in one function

function calculate() {
	// if the operator used was the multiplication, then multiply what is stored in memory with the current value

	switch (true) {
		case resultValue === 1:
			currentValue = parseFloat(memory) * parseFloat(currentValue);
			break;
		case resultValue === 2:
			currentValue = parseFloat(memory) / parseFloat(currentValue);
			break;
		case parseFloat(currentValue) !== 0:
			currentValue = "Error";
			break;
		case resultValue === 3:
			currentValue = parseFloat(memory) + parseFloat(currentValue);
			break;
		case resultValue === 4:
			currentValue = parseFloat(memory) - parseFloat(currentValue);
	}
	// if (resultValue === 1) {
	// 	currentValue = parseFloat(memory) * parseFloat(currentValue);
	// }

	// // if the operator is the division one, then multiply the value stored in memory with the current value
	// if (resultValue === 2) {
	// 	if (parseFloat(currentValue) !== 0) {
	// 		// unless it is trying to divide by zero
	// 		currentValue = parseFloat(memory) / parseFloat(currentValue);
	// 	} else {
	// 		currentValue = "Error";
	// 	}
	// }
	// // if the operation used was addition, then add what is stored in memory with the current value
	// if (resultValue === 3) {
	// 	currentValue = parseFloat(memory) + parseFloat(currentValue);
	// }
	// // if the operation used was subtraction, then subtract what is stored in memory with the current value
	// if (resultValue === 4) {
	// 	currentValue = parseFloat(memory) - parseFloat(currentValue);
	// }

	// reset the value stored in the memory and the current value and force the current value to a string after completing the calculation

	currentValue = currentValue + "";
	resultValue = 0;
	memory = "0";

	display.innerHTML = currentValue;
}