script.js

/*
*caldulate() is called when the user clicks any operation button.
*It reads the two numbers, validates them, does the maths, and displays the result.
*The paramets "opertion" is a string: 'add', 'subtract', 'multiplt', or 'divide'.
*/
function calculate(operation) {
    // 1. GET REFFERENCES TO TML ELEMENTS ---
    var resultDisplay = document.getElementById("result");
    var errorDisplay = document.getElementById("error");

    // Clear any previous error message
    errorDisplay.textContent = "";
    resultDisplay.textContent = "";

    // 2. READ USER'S INPUT ---
    var input1 = document.getElementById("number1").value;
    var input2 = document.getElementById("number2").value;

    // 3. DATA VALIDATION ---
    // CHeck if either field is empty
    if (input1 === "" || input2 === "") {
        errorDisplay.textContent = "Please enter both numbers.";
        return;         //stop the function here
    }

    //Convert the text input into actual numbers
    var num1 = Number(input1);
    var num2 = Number(input2);

    // Check if the conversion was successful (i.e., the input was a valid number)
    if (isNaN(num1)) {
        errorDisplay.textContent = "The first input is not a valid number. Please enter valid numbers.";
        return;         //stop the function here
    }

    if (isNaN(num2)) {
        errorDisplay.textContent = "The second input is not a valid number. Please enter valid numbers.";
        return;         //stop the function here
    }

    // 4. PERFORM THE ARITHMETIC ---

    var answer;

    if (operation === "add") {
        answer = num1 + num2;

    } else if (operation === "subtract") {
        answer = num1 - num2;

    } else if (operation === "multiply") {
        answer = num1 * num2;

    }else if (operation === "divide") {
        // Extra check: cannot divide by zero
        if (num2 === 0) {
            errorDisplay.textContent = "Cannot divide by zero. Please enter a non-zero number for the second input.";
            return;         //stop the function here
        }
        answer = num1 / num2;
    }
        // --- 5. SHOW THE RESULT ---

        resultDisplay.textContent = answer + " :)";
}