javascript if else calculator [duplicate]

what's the problem with dividing by zero? Even if I write a number bigger than zero I receive alert.

const firstNumber = Number(prompt('Enter first number'));
const operator = prompt('+ - * /');
const secondNumber = Number(prompt('Enter second number'));

if(operator === '+') {
    console.log(firstNumber + secondNumber);
} else if(operator === '-') {
    console.log(firstNumber - secondNumber);
} else if(operator === '*') {
    console.log(firstNumber * secondNumber);
} else if(operator === '/') {
        if (firstNumber || secondNumber === 0) {
              alert("You can't divide by zero.");
               
           } else if (firstNumber || secondNumber > 0) {
               console.log(firstNumber / secondNumber);
                
           }
} else {
    console.log("You didn't enter a number");
}