index.js

const form=document.querySelector('form');
const name=document.querySelector('#name');
const cost=document.querySelector('#cost');
const error=document.querySelector('#error');

form.addEventListener('submit',(e)=>{
e.preventDefault()
if(name.value && cost.value){
const item={
    name:name.value,
    cost:parseInt(cost.value)
}
db.collection('expenses').add(item).then(res=>{
    error.textContent="";
    name.value="";
    cost.value="";
})
}
else{
   error.textContent="Please enter values before submitting" 
}
})