如何在firebase事务api上处理空值

Question

我有一个函数，是递减用户信用的firebase实时数据库值与事务。正如Firebase事务API调用中所建议的，当前数据为空事务当前值偶尔返回为空。

function charge(cost, description) {
  return new Promise((resolve, reject) => {
    const creditRef = db.ref(`credits/${userid}`)
    ref.transaction(function(current) {
      console.log(`${description} current value: ${current}`)
      if (current === null) {
        console.log(`${description} returns 0 `)
        return 0
      }
      if (cost > current || current === 0) {
        //not enough credits return without committing
        console.log(`${description} aborts `)
        return
      }
      //commit the new credit value
      console.log(`${description} returns ${current} - ${cost}`)
      return current - cost
    },
    (error, commited, snapshot) => {
      if (error) {
        reject(error)
      }
      else {
        if (commited) {
            //user has enough credits
            resolve()
        }
        else {
            //not enough credits
            reject('no commit')
        }
    }
  })
}

// User has 5 credits 
charge(3, 'first call').then(() => console.log('first call success')
// transaction function returns 0 since current value is null
charge(2, 'second call').then(() => console.log('second call success')

第二次调用当前值：0

第二次调用中止

首次调用电流值：5

第一次呼叫返回5-3

首次呼叫成功

第二次调用不提交

因此，当用户有足够的信用时，第二个收费电话最终没有通过。处理firebase事务空值情况的正确方法是什么？

Answer 1

const ref = firebase.database().ref(`59571450`)
function charge(cost, description) {
  return new Promise((resolve, reject) => {
    ref.transaction(function(current) {
      console.log(`${description} current value: ${current}`);
      if (current === null) {
        console.log(`${description} returns null`)
        return null;
      }
      if (cost > current) {
        //not enough credits return without committing
        console.log(`${description} aborts `)
        return
      }
      //commit the new credit value
      console.log(`${description} returns ${current} - ${cost}`)
      return current - cost
    },
    (error, commited, snapshot) => {
      if (error) {
        reject(error)
      }
      else {
        if (commited) {
            //user has enough credits
            resolve()
        }
        else {
            //not enough credits
            reject('no commit')
        }
      }
    });
  });
}
                     

ref.set(5).then(() => {
  charge(3, 'first call').then(() => console.log('first call success'))
  charge(2, 'second call').then(() => console.log('second call success'))
})

第二次呼叫成功

有关此代码的工作版本，请参见:https://jsbin.com/xefebul/1/edit?js，console