script.js

let expression = "0";

const DISPLAY = document.getElementById("user-input");
const RESULT = document.getElementById("result");

function appendToDisplay(value) {
	// Check for existing decimal in the current number
	if (value === "." && /\.\d*$/.test(expression)) {
		return;
	} else if (isOperator(value)) {
		// Replace previous operators
		expression = expression.replace(/[-+*/]$/, "");
	}

	// special case: Prevent appending a zero in front when any number is pressed
	if (expression === "0" && !isOperator(value)) {
		expression = value;
	} else {
		expression += value;
	}

	updateDisplay(expression);
}

function isOperator(value) {
	return value === "+" || value === "-" || value === "*" || value === "/";
}

function updateResult() {
	RESULT.value = expression;
}

function clearAllDisplay() {
	expression = "0";
	updateResult();
	updateDisplay(expression);
}

function deleteDisplay() {
	expression = expression.slice(0, -1);

	if (expression == "") {
		expression = "0";
	}

	updateDisplay(expression);
}

function updateDisplay(value) {
	//append 0 to expression if the first value is an operator i.e., +2 to be displayed as 0 + 2
	if (RESULT.value == "0" && isOperator(value.charAt(0))) {
		value = appendZero(value);
	}

	DISPLAY.value = transformOperators(value);
}

function appendZero(value) {
	return "0" + value;
}

function transformOperators(value) {
	if (value.includes("*") || value.includes("/")) {
		value = value.replace(/\*/g, "×").replace(/\//g, "÷");
	}
	return value;
}

function calculateResult() {
	try {
		const result = math.evaluate(expression);

		if (Number.isInteger(result)) {
			expression = result.toString(); // display whole numbers without trailing zeros
		} else {
			const resultString = result.toString();
			const decimalIndex = resultString.indexOf(".");
			// Round up long decimal values to 5 decimal places.
			if (
				decimalIndex !== -1 &&
				resultString.length - decimalIndex > 6
			) {
				expression = (
					Math.round(result * 100000) / 100000
				).toString();
			} else if (resultString === "Infinity") {
				alert("You cannot divide by Zero!");
				clearAllDisplay();
			} else {
				expression = resultString;
			}
		}
		updateResult();
	} catch (error) {
		alert("Invalid expression");
		clearAllDisplay();
	}
}

document.onload = clearAllDisplay();