Что не так с js кодом?

 a = '' 
b = '' 
sign = '' 
finish = false 
 
digit = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '.'] 
 
action = ['+', '-', 'X', '/'] 
 
out = document.querySelector('.calc-screen p') 
 
function ClearAll() { 
	a = '' 
	b = '' 
	sign = '' 
	finish = false 
	out.textContent = 0 
} 
 
document.querySelector('.ac').onclick = ClearAll 
 
document.querySelector('.calc-buttons').onclick = event => { 
	if (!event.target.classList.contains('.btn')) return 
	if (event.target.classList.contains('.ac')) return 
 
	out.textContent = '' 
 
	key = event.target.textContent 
 
	if (digit.includes(key)) { 
		a += key 
		console.log(a, sign, b) 
		out.textContent = a 
	} 
 
	if (action.includes(key)) { 
		sign += key 
		console.log(a, sign, b) 
		out.textContent = sign 
	} 
}