您应该使用方括号notation，如下所示：

var db = firebase.firestore()

    db.collection('Collection_Id').doc('Doc_Id').get().then(function (doc) {
        if (doc.exists) {
            const monthNames = ["January", "February", "March", "April", "May", "June",
                "July", "August", "September", "October", "November", "December"]

            const date = new Date()
            const currentMonth = monthNames[date.getMonth()]
            location.airTemp = doc.data()[currentMonth].airTemp

        } else {
            // doc.data() will be undefined in this case
            console.log("No such document!")
        }
    });