从数组中提取对象时减少返回 NaN 的函数 - MongoDB 数据库，React 前端

Question

I have the following Schema:

const SubmitDebtSchema = new Schema ({
  balance: [{
    balanceDate: Date,
    newBalance: Number
  }],
});

I am attempting to loop through my database entries, pull the 'newBalance' out from each object in the balance array, and then reduce / sum them together.

However, it is returning 'NaN' - and I can't figure out why.

Here is my Axios call to get the data:

  componentDidMount() {

    axios.get("/api/fetch/fetchDebtCards")
    .then((response) => {
      this.setState({
        debts: response.data
      })
      console.log(this.state.debts.balance.newBalance)
    })
  }

The console log in there successfully retrieves the database entries.

And here is my reduce function:

const sumBalance = this.state.debts.reduce(function(previousValue, currentValue) {
          return (
            previousValue + currentValue.balance.newBalance
          )
        }, 0)

You can see, I'm attempting to tap into 'balance.newBalance' to access the newBalance within each of the balance objects.

Can anyone point out what I'm doing wrong?

EDIT: My console log, with two entries. What I want to do is get the newBalance array.length -1 from these, and sum them together by reducing.

[Log] Array

0 Object

balance: [{_id: "5fbbddd1077c56000828973c", balanceDate: "2020-11-23T16:05:36.124Z", newBalance: 400}]

1 Object

balance: [{_id: "5fbc06f58b2f98000865df54", balanceDate: "2020-11-23T19:01:07.789Z", newBalance: 300}] (1)

Answer 1

if "console.log(this.state.debts.balance.newBalance)" works then debts is not an array, how are you using map on debts then? Map can only be used on arrays.

I'm not sure how your debts object/array is exactly. Maybe a log of it would be helpful.

If it is an array, then this might work.

const sumBalance = this.state.debts.map(x => x.balance).flat().reduce((a,b) => a + b.newBalance, 0)

whereas if it's an object, then this might work

const sumBalance = this.state.debts.balance.reduce((a,b) => a+b.newBalance, 0)

If none of these work, just log "this.state.debts" and let us see what you have there.

Edit: Ok so you only need the last values of the balance arrays (the latest balance), something like this?

const sumBalance = this.state.debts.map(x => x.balance[x.balance.length-1].newBalance).reduce((a,b) => a + b, 0)