CODEWITHSUNDEEP
javascriptarraysecmascript-6reduce
Neil Merton
Neil Merton

Reputation: 573

Javascript array reduce return zero instead of undefined

I've got an example array that I'm trying to reduce by the counts of the occurrence of a key (sentiment in this example):

const sentimentLog = [
  {
    id: 1,
    createdOn: new Date('2020-02-13'),
    sentiment: 1
  },
  {
    id: 2,
    createdOn: new Date('2020-02-12'),
    sentiment: 1
  },
  {
    id: 3,
    createdOn: new Date('2020-02-12'),
    sentiment: 2
  },
  {
    id: 4,
    createdOn: new Date('2020-02-11'),
    sentiment: 3
  },
  {
    id: 5,
    createdOn: new Date('2020-02-11'),
    sentiment: 2
  },
  {
    id: 6,
    createdOn: new Date('2020-02-10'),
    sentiment: 1
  },
  {
    id: 7,
    createdOn: new Date('2020-02-10'),
    sentiment: 2
  },
  {
    id: 8,
    createdOn: new Date('2020-02-09'),
    sentiment: 1
  }
]

I'm using:

const sentimentGrouped = (sentiments) => {
  return sentiments.reduce((hash, { sentiment }) => {
    hash[sentiment] = (hash[sentiment] || 0) + 1
    return hash
  }, [])
}

And it's nearly there. What I can't figure out is how to replace undefined when there's no sentiment scores of 0 (which is a possibility).

console.log('sentimentGrouped', sentimentGrouped(sentimentLog))

The above produces:

"sentimentGrouped" [undefined, 4, 3, 1]

Whereas I'd like:

"sentimentGrouped" [0, 4, 3, 1]

What am I missing?

Thanks in advance.

Edit: I'll elaborate a bit further, there's 4 scores that will be returned (0 to 3). The data returned will be based on a date range. So there may be instances where there'll be no 1s returned, similarly no 3s returned by a different date range.

Upvotes: 0

Views: 568

Answers (1)

Nicholas Tower
Nicholas Tower

Reputation: 84912

The issue is that if you never touch an element of the array, then it stays as a hole in the array, which means it's treated as undefined. Since you know the length of the array i would just prefill the array with zeros. Any sentiment score that does occur will be incremented. Any one that doesn't will stay with its initial value.

return sentiments.reduce((hash, { sentiment }) => {
  hash[sentiment] = hash[sentiment] + 1
  return hash
}, [0, 0, 0, 0])

Upvotes: 4

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