works, but allows the first input to be an operator(+,-,*,/,=). How can I make the first input be number only?

const buttons = document.querySelectorAll('button');
const display = document.querySelector('.display');

added a variable to distinguish the operators

const operator = ['+', '-', '*', '/'];

buttons.forEach(function(button) {
  button.addEventListener('click', calculate);
});

function calculate(event) {
const clickedButtonValue = event.target.value;

tried this, to no effect

if (display.value[0] = operator) {
  display.value = '';
}

else if (clickedButtonValue === '=' && display.value !== '') {
  display.value = eval(display.value)
} else if (clickedButtonValue === 'C') {
  display.value = ''
} else {
  display.value += clickedButtonValue;
}


}