How can an array be grouped by two properties?

Question

Ex:

const arr = [{
  group: 1,
  question: {
    templateId: 100
  }
}, {
  group: 2,
  question: {
    templateId: 200
  }
}, {
  group: 1,         


        

Answer 1

@Bergi's answer is really practical but I'll show you how building a multi-value "key" can be possible using JavaScript primitives – don't take this to mean Bergi's answer is bad in anyway; in fact, it's actually a lot better because of it's practicality. If anything, this answer exists to show you how much work is saved by using an approach like his.

I'm going to go over the code bit-by-bit and then I'll have a complete runnable demo at the end.

---

compound data equality

Comparing compound data in JavaScript is a little tricky, so we're gonna need to figure out a way around this first:

console.log([1,2] === [1,2]) // false

I want to cover a solution for the multi-value key because our entire answer will be based upon it - here I'm calling it a CollationKey. Our key holds some value and defines its own equality function which is used for comparing keys

const CollationKey = eq => x => ({
  x,
  eq: ({x: y}) => eq(x, y)
})

const myKey = CollationKey (([x1, x2], [y1, y2]) =>
  x1 === y1 && x2 === y2)

const k1 = myKey([1, 2])
const k2 = myKey([1, 2])
console.log(k1.eq(k2)) // true
console.log(k2.eq(k1)) // true

const k3 = myKey([3, 4])
console.log(k1.eq(k3)) // false

---

wishful thinking

Now that we have a way to compare compound data, I want to make a custom reducing function that uses our multi-value key to group values. I'll call this function collateBy

// key = some function that makes our key
// reducer = some function that does our reducing
// xs = some input array
const collateBy = key => reducer => xs => {
  // ...?
}

// our custom key;
// equality comparison of `group` and `question.templateId` properties
const myKey = CollationKey ((x, y) =>
  x.group === y.group
    && x.question.templateId === y.question.templateId)

const result =
  collateBy (myKey) // multi-value key
            ((group=[], x) => [...group, x]) // reducing function: (accumulator, elem)
            (arr) // input array

So now that we know how we want collateBy to work, let's implement it

const collateBy = key => reducer => xs => {
  return xs.reduce((acc, x) => {
    const k = key(x)
    return acc.set(k, reducer(acc.get(k), x))
  }, Collation())
}

---

Collation data container

Ok, so we were being a little optimistic there too using Collation() as the starting value for the xs.reduce call. What should Collation be?

What we know:

• someCollation.set accepts a CollationKey and some value, and returns a new Collation
• someCollation.get accepts a CollationKey and returns some value

Well let's get to work!

const Collation = (pairs=[]) => ({
  has (key) {
    return pairs.some(([k, v]) => key.eq(k))
  },
  get (key) {
    return (([k, v]=[]) => v)(
      pairs.find(([k, v]) => k.eq(key))
    )
  },
  set (key, value) {
    return this.has(key)
      ? Collation(pairs.map(([k, v]) => k.eq(key) ? [key, value] : [k, v]))
      : Collation([...pairs, [key, value]])
  },
})

---

finishing up

So far our collateBy function returns a Collation data container which is internally implemented with an array of [key, value] pairs, but what we really want back (according to your question) is just an array of values

Let's modify collateBy in the slightest way that extracts the values – changes in bold

const collateBy = key => reducer => xs => {
  return xs.reduce((acc, x) => {
    let k = key(x)
    return acc.set(k, reducer(acc.get(k), x))
  }, Collation()).values()
}

So now we will add the values method to our Collation container

values () {
  return pairs.map(([k, v]) => v)
}

---

runnable demo

That's everything, so let's see it all work now – I used JSON.stringify in the output so that the deeply nested objects would display all content

// data containers
const CollationKey = eq => x => ({
  x,
  eq: ({x: y}) => eq(x, y)
})

const Collation = (pairs=[]) => ({
  has (key) {
    return pairs.some(([k, v]) => key.eq(k))
  },
  get (key) {
    return (([k, v]=[]) => v)(
      pairs.find(([k, v]) => k.eq(key))
    )
  },
  set (key, value) {
    return this.has(key)
      ? Collation(pairs.map(([k, v]) => k.eq(key) ? [key, value] : [k, v]))
      : Collation([...pairs, [key, value]])
  },
  values () {
    return pairs.map(([k, v]) => v)
  }
})

// collateBy
const collateBy = key => reducer => xs => {
  return xs.reduce((acc, x) => {
    const k = key(x)
    return acc.set(k, reducer(acc.get(k), x))
  }, Collation()).values()
}

// custom key used for your specific collation
const myKey =
  CollationKey ((x, y) =>
    x.group === y.group
      && x.question.templateId === y.question.templateId)

// your data
const arr = [ { group: 1, question: { templateId: 100 } }, { group: 2, question: { templateId: 200 } }, { group: 1, question: { templateId: 100 } }, { group: 1, question: { templateId: 300 } } ]

// your answer
const result =
  collateBy (myKey) ((group=[], x) => [...group, x]) (arr)

console.log(result)
// [
//   [
//     {group:1,question:{templateId:100}},
//     {group:1,question:{templateId:100}}
//   ],
//   [
//     {group:2,question:{templateId:200}}
//   ],
//   [
//     {group:1,question:{templateId:300}}
//   ]
// ]

---

summary

We made a custom collation function which uses a multi-value key for grouping our collated values. This was done using nothing but JavaScript primitives and higher-order functions. We now have a way to iterate thru a data set and collate it in an arbitrary way using keys of any complexity.

If you have any questions about this, I'm happy to answer them ^_^