我认为这是一个纯粹的JS问题，我的解决方案是在从firestore中获取对象后，对对象数组使用JS reduce函数。

function getSum(total, num) {
        return (total + parseInt(num.equipment));
    }

useEffect(() => {
    var firestoreArray = [...]; //this is populated with the array of objects 
// from firestore when you fetch. format is {name: "shovel", equipment: "12" }

  var total = firestoreArray.reduce(getSum, 0); //reduce function where the 0 is the starting 
//point of the sum. the getSum function is adding by parsing to Integer first for all the 
//objects "equipment" attribute.

}, [firestoreArray]}