calculator.js

let runningTotal = 0;
let buffer = "0";
let previousOperator = null;

const screen = document.querySelector(".screen");

function buttonClick(value) {
    if (isNaN(value)) {
        // This is not a number.
        handleSymbol(value)
    } else {
        // This is a number.
        handleNumber(value);
    }
    screen.innerText = buffer;
}

function handleSymbol(symbol) {
    switch (symbol) {
        case 'C':
            buffer = "0";
            runningTotal = 0;
            break;
        case '=':
            if (previousOperator === null) {
                // Need two numbers tio do math.
                return;
            }
            flushOperation(+buffer);
            previousOperator = null;
            buffer = runningTotal;
            runningTotal = 0;
            break;
        case '←':
            if (buffer.length === 1) {
                buffer = "0"
            } else {
                buffer = buffer.substring(0, buffer.length - 1);
            }
            break;
        case '+':
        case '−':   
        case '×':
        case '÷':
            handleMath(symbol);
            break;
    }
}

function handleMath(symbol) {
    if (buffer === "0") {
        // Do nothing.
        return;
    }

    const intBuffer = +buffer;

    if (runningTotal === 0) {
        runningTotal = intBuffer;
    } else {
        flushOperation(intBuffer);
    }
    
    previousOperator = symbol;

    buffer = "0";
}

function flushOperation(intBuffer) {
    if (previousOperator === "+") {
        runningTotal += intBuffer;
    } else if (previousOperator === "−") {
        runningTotal -= intBuffer;
    } else if (previousOperator === "×") {
        runningTotal *= intBuffer;
    } else {
        runningTotal /= intBuffer;
    }
}

function handleNumber(numberString) {
    if (buffer === "0") {
        buffer = numberString;
    } else {
        buffer += numberString;
    }
}

function init() {
    document.querySelector('.calc-buttons')
        .addEventListener('click', function(event) {
            buttonClick(event.target.innerText);
        })
}

init()