Javascript – JS Vanilla Calculator – change operator

Currently building a simple calculator with vanilla JS.
All buttons and calculations work but i needed help with switching operators

When the user performs an operation it works just fine, but let’s say the user pressed 2 and the operator "+" instead of "-" like he wanted, so instead of clearing the screen he presses "-" right after (so "2" then "+" then "-"), the screen displays "NaN -" instead of just switching the operator.

How would I fix that?

let currentOperand = '';
let previousOperand = '';
let currentOperation = undefined;
let isEqualsPressed = false;


numberBtns.forEach(btn => {
    btn.addEventListener('click', () => {
        inputNumber(btn.innerHTML)
    })
}) 

operationBtns.forEach(btn => {
    btn.addEventListener('click', () => {
        isEqualsPressed = true;
        chooseOperation(btn.innerHTML)
    })
}) 

allClear.addEventListener('click', clear);
del.addEventListener('click', deleteLastNumber);
equals.addEventListener('click', evaluate);

function clear() {
    currentOperand = '';
    previousOperand = '';
    currentOperandScreen.textContent = '';
    previousOperandScreen.textContent = '';
    operator = undefined;
} 

function deleteLastNumber() {
    currentOperandScreen.innerHTML = currentOperandScreen.innerHTML.slice(0, -1)
} 

function inputNumber(number) {
    if(currentOperandScreen.innerHTML.includes(".") && number === ".") return;

    if(currentOperandScreen.innerHTML.length < 16)
    currentOperandScreen.innerHTML = (currentOperandScreen.innerHTML).toString() + number.toString();
} 

function chooseOperation (operator) {
    if (previousOperand !== '') {
        evaluate()
    }  
    previousOperand = currentOperandScreen.innerHTML;
    currentOperation = operator;
    currentOperandScreen.innerHTML = '';
    previousOperandScreen.innerHTML = `${previousOperand} ${operator}`;
}

function evaluate() {
    let result;
    previousOperand = parseFloat(previousOperand)
    currentOperand = parseFloat(currentOperandScreen.innerHTML);

    switch (currentOperation) {
        case "+":
            result = add(previousOperand, currentOperand);
            break;
        case "-":
            result = subtract(previousOperand, currentOperand);
            break;
        case "÷":
            if(currentOperand === 0) {
                result = "Error"
            } else
            {result = divide(previousOperand, currentOperand)};
            break;
        case "x":
            result = multiply(previousOperand, currentOperand);
            break;
        default:
            return;
    }

    if (isEqualsPressed == true) {
        previousOperandScreen.innerHTML = `${previousOperand} ${currentOperation} ${currentOperand} =`
        currentOperandScreen.innerHTML = result;
    }

    currentOperand = result;
    currentOperandScreen.innerHTML = result;
    currentOperation = undefined;
    previousOperand = '';
}

thanks in advance!