How to reduce and map an object array?

Question

How do I reduce this object array and and map it to a new array?

My data:

var objArray = 
  [{
    state: 'NY',
    type: 'A',
    population: 100
  },
  {
    state: 'NY',
    type: 'A',
    population: 300
  },
  {
    state: 'NY',
    type: 'B',
    population: 200
  },
  {
    state: 'CA',
    type: 'A',
    population: 400
  },
  {
    state: 'CA',
    type: 'A',
    population: 400
  }];

If an entry has the same state AND type I need to combine it into a single entry and sum their populations.

Finally I need to map it to an array in this format.

 var outputArray = [ ['A', 'NY', 400 ], ['B', 'NY', 200], ['A', 'CA', 800] ]

You need to show what you have tried. Stackoverflow isn't a free code writing or tutorial service. The objective here is to help you fix your code — charlietfl
– charlietfl, Commented Oct 11, 2017 at 22:41

Max · Accepted Answer · 2017-10-11 23:07:47Z

6

Well, first you'd want to reduce it. This can be done like so...

objArray.reduce((prev, obj) => {
    if(1 + (indx = prev.findIndex(oldObj => oldObj.type === obj.type && oldObj.state === obj.state))) {
        prev[indx].population += obj.population;
    } else {
        prev.push({...obj})
    }
    return prev;
}, [])

This takes the array being gathered, and modifies it in some way and returns it in the reduce callback. It will either modify an existing value's population, if it can find one with the correct state and type; or it will append a new object to the end of the array.

Now you need to map it.

.map(obj => [ obj.type, obj.state, obj.population ])

edited Oct 11, 2017 at 23:07

answered Oct 11, 2017 at 22:46

Max

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10 Comments

Jaromanda X Over a year ago

note, that this code mutates the original array too

Max Over a year ago

It mutates prev, but that's different each iteration and is intentionally mutated. I don't think objArray is, though -- right?

Jaromanda X Over a year ago

console.log(objArray) - you'll see that it has changed population in items 0 and 3

Jaromanda X Over a year ago

you push obj - objects are pass by reference - so, when you update prev[0].population, it's the same object as objArray[0], so objArray[0].population is the same property as prev[0].population

Max Over a year ago

@jm22 When you put a key without a value in a JS object literal, it's a shorthand for obj: obj, i.e. pushing itself. You have to use the spread op to push its contents, not itself

|

Derek Brown · Accepted Answer · 2017-10-11 22:58:12Z

1

I would recommend using the lodash package (it is an extremely common package in Javascript applications). It adds a lot of great functions for manipulating arrays. This post explains how to sum values on groups. You will need to modify this answer slightly to account for your two parameters, which you can do by changing the groupBy command to this:

_.groupBy(objArray, function(val){
    return val.state + "#" + val.type
})

answered Oct 11, 2017 at 22:58

Derek Brown

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Comments

Nothing · Accepted Answer · 2017-10-11 23:11:16Z

1

You could try something like this:

var arr = Object.keys(objArray).map(function (key) { return Object.keys(objArray[key]).map(function (key) { return [[objArray[0].state, objArray[0].type, objArray[0].population],[objArray[1].state, objArray[1].type, objArray[1].population]]}); })[0][0];

answered Oct 11, 2017 at 23:11

Nothing

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Comments

Mark · Accepted Answer · 2017-10-11 23:44:57Z

1

If you know state and type will never have a certain character such as '_' you can make a key out of state and type such as 'NY_A' - a little like a composite key in a DB. Then you just create an object with these keys, add the populations and then pull them apart into an array:

Object.entries(
    objArray.reduce((acc,curr) => (
    acc[curr.type + '_' + curr.state] = curr.population + (acc[curr.type + '_' + curr.state] || 0)
    , acc), {}))
.map(item => [...item[0].split('_'), item[1]])

answered Oct 11, 2017 at 23:44

Mark

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Comments

Leon Li · Accepted Answer · 2017-10-12 01:41:09Z

const _ = require('lodash')

const input = [
  { state: 'NY', type: 'A', population: 100 },
  { state: 'NY', type: 'A', population: 300 },
  { state: 'NY', type: 'B', population: 200 },
  { state: 'CA', type: 'A', population: 400 },
  { state: 'CA', type: 'A', population: 400 }
]

const expected = [
  ['A', 'NY', 400],
  ['B', 'NY', 200],
  ['A', 'CA', 800]
]

const output = _(input)
  .groupBy(({ state, type }) => [state, type].join(':'))
  .mapValues((states) => states.reduce((total, { population }) => total + population, 0))
  .map((population, stateType) => {
    const [state, type] = stateType.split(':')
    return [type, state, population]
  })
  .value()

console.log(expected)
console.log(output)

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