下面的代码应该可以完成这个任务，注意Cloud Firestore触发器（users/{userId}/wordScores/{scoreDocId}）中的路径：它指向一个文档，而不是一个集合。更多信息在documentation。

exports.calculateAccuracy = functions.firestore
    .document('users/{userId}/wordScores/{scoreDocId}')
    .onUpdate((change, context) => {

        const newValue = change.after.data();
        const previousValue = change.before.data();

        if ((newValue.attempts !== previousValue.attempts) || (newValue.successes !== previousValue.successes)) {
            return change.after.ref.update({
                accuracy: newValue.successes / newValue.attempts
            });

        } else {
            return null;
        }

    });