Grouping an array of objects by key

We are going to implement a groupBy function, which when given a collection and a key returns a new Map() grouped by that key.

For example given this input:

const blogPosts = [
  { category: 'javascript', author: 'Alex', title: 'Usecases for reduce' },
  { category: 'php', author: 'Alex', title: 'Getting started with phpunit' },
  { category: 'javascript', author: 'Ben', title: 'Maps and Sets' },
]

const blogPostsByCategory = ???

blogPostsByCategory.forEach(category => console.table(category))

It should produce this output (console.table() is a hidden gem):

┌─────────┬──────────────┬────────┬───────────────────────┐
│ (index) │   category   │ author │         title         │
├─────────┼──────────────┼────────┼───────────────────────┤
│    0    │ 'javascript' │ 'Alex' │ 'Usecases for reduce' │
│    1    │ 'javascript' │ 'Ben'  │    'Maps and Sets'    │
└─────────┴──────────────┴────────┴───────────────────────┘
┌─────────┬──────────┬────────┬────────────────────────────────┐
│ (index) │ category │ author │             title              │
├─────────┼──────────┼────────┼────────────────────────────────┤
│    0    │  'php'   │ 'Alex' │ 'Getting started with phpunit' │
└─────────┴──────────┴────────┴────────────────────────────────┘

We are going to do this step by step, starting with the reduce() method, diving quickly into higher-order functions, then examining the Map data structure. Finally, we put everything together for our groupBy function!

reduce()

The reduce() method takes an array and combines it into a single value. It does that by executing a reducer function and accumulating the result.

The most common example is summing all the numbers in an array.

const numbers = [1, 2, 3]

const add = (a, b) => a + b

const sumOfNumbers = numbers.reduce(add)

console.log(sumOfNumbers)

6

Higher-Order Functions

Higher Order Functions are functions, that can return a function or take a function as an argument. map(), reduce() and filter() are higher-order functions.

Currying

Curried functions are functions that return a function which takes one argument.

const addCurried = a => b => a + b

//    add2 is a function, which takes exactly one argument now
const add2 = addCurried(2)
//    addCurried = a => b => a + b
//    add2 =       2 => b => 2 + b
//    add2 =            b => 2 + b

const sumOfnumbersPlus2 = numbers.map(add2).reduce(add)

console.log(sumOfnumbersPlus2)

12

Map<Key,Value>

A Map is an object which maps a given key to a value. The key can be anything, a String, a Number or even an Object. The same is true for the value: It could also be an Array, which will become important later.

When iterating over a map, the order is the insertion order of the values.

A Map of names starting with a certain character

const namesStartingWithA = ['Alex', 'Adrian']
const namesStartingWithB = ['Ben']
const namesStartingWithC = ['Charlie', 'Carl']

const nameMap = new Map([
  ['A', namesStartingWithA],
  ['B', namesStartingWithB],
  ['C', namesStartingWithC],
])

console.table(nameMap)

┌───────────────────┬─────┬───────────────────────┐
│ (iteration index) │ Key │        Values         │
├───────────────────┼─────┼───────────────────────┤
│         0         │ 'A' │ [ 'Alex', 'Adrian' ]  │
│         1         │ 'B' │       [ 'Ben' ]       │
│         2         │ 'C' │ [ 'Charlie', 'Carl' ] │
└───────────────────┴─────┴───────────────────────┘

Retrieving only the names starting with "A"

We use map.get(key) to do that.

console.log(nameMap.get('A'))

[ 'Alex', 'Adrian' ]

Adding a new name

We use map.set(newValue) to set a new value for that key.

We have to create a new array, where our new name is added to the old names. We set this new array as our value for the key.

nameMap.set('A', [...namesStartingWithA, 'Alf'])
console.log(nameMap.get('A'))

[ 'Alex', 'Adrian', 'Alf' ]

Creating a helper function to append values to an array

Getting the array via map.get() can get tedious after a while. Luckily we can create a higher-order function, which helps to append a value to an existing array for that key, or if it doesn't exist, creating a new one.

We concatenate the old array with the new value using the spread syntax.

// Higher order function, fn(map) -> fn(key) -> fn(value)
const appendValueToArray = map => key => value => {
  // if the map has that key, use that value as array, if not create a new one
  const currentValues = map.get(key) || []

  return map.set(key, [...currentValues, value])
}

appendValueToArray(nameMap)('A')('Albert')
console.log(nameMap.get('A'))

[ 'Alex', 'Adrian', 'Alf', 'Albert' ]

Grouping blogposts by their category

We now have all our tools needed to create a reduce function, which can group our blogposts by their category. Let's get started by "creating" some blog posts.

const blogPosts = [
  { category: 'javascript', author: 'Alex', title: 'Usecases for reduce' },
  { category: 'php', author: 'Alex', title: 'Getting started with phpunit' },
  { category: 'javascript', author: 'Ben', title: 'Maps and Sets' },
]

First, we create our reducer function, making use of our appendValueToArray() method. We get the category of the blog post by doing a destructuring assignment.

// returns the map with our new value appended for the array of the category key
const groupByCategory = (blogPostCategoryMap, blogPost) => {
  const { category } = blogPost
  const addBlogPostsToCategory = appendValueToArray(blogPostCategoryMap)

  return addBlogPostsToCategory(category)(blogPost)
}

Now the missing reduce() call to get our blog post by category is just a cute little line. Note that we have to provide our initial value for our reducer of an empty new Map().

const blogPostsByCategory = blogPosts.reduce(groupByCategory, new Map())

We can then iterate with map.forEach(value, key) over the map to get a pretty output of blogPostsByCategory.

blogPostsByCategory.forEach((posts, category) => {
  console.log(`Category: ${category}`)
  console.table(posts)
})

Category: javascript
┌─────────┬──────────────┬────────┬───────────────────────┐
│ (index) │   category   │ author │         title         │
├─────────┼──────────────┼────────┼───────────────────────┤
│    0    │ 'javascript' │ 'Alex' │ 'Usecases for reduce' │
│    1    │ 'javascript' │ 'Ben'  │    'Maps and Sets'    │
└─────────┴──────────────┴────────┴───────────────────────┘
Category: php
┌─────────┬──────────┬────────┬────────────────────────────────┐
│ (index) │ category │ author │             title              │
├─────────┼──────────┼────────┼────────────────────────────────┤
│    0    │  'php'   │ 'Alex' │ 'Getting started with phpunit' │
└─────────┴──────────┴────────┴────────────────────────────────┘

Implementing a generic groupBy(key)

We can further generalize this by implementing a generic groupBy method, which groups by any given key.

const groupByKey = key => (mapGroupedByKey, entry) => {
  const addEntryToKey = appendValueToArray(mapGroupedByKey)(entry[key])
  return addEntryToKey(entry)
}

const groupedByKey = (array, key) => array.reduce(groupByKey(key), new Map())

const blogPostsByKey = key => groupedByKey(blogPosts, key)

const blogPostsByAuthor = blogPostsByKey('author')

const blogPostsByCategoryNew = blogPostsByKey('category')

blogPostsByAuthor.forEach(posts => console.table(posts))

┌─────────┬──────────────┬────────┬────────────────────────────────┐
│ (index) │   category   │ author │             title              │
├─────────┼──────────────┼────────┼────────────────────────────────┤
│    0    │ 'javascript' │ 'Alex' │     'Usecases for reduce'      │
│    1    │    'php'     │ 'Alex' │ 'Getting started with phpunit' │
└─────────┴──────────────┴────────┴────────────────────────────────┘
┌─────────┬──────────────┬────────┬─────────────────┐
│ (index) │   category   │ author │      title      │
├─────────┼──────────────┼────────┼─────────────────┤
│    0    │ 'javascript' │ 'Ben'  │ 'Maps and Sets' │
└─────────┴──────────────┴────────┴─────────────────┘

Don't worry if you struggle to wrap around your head around these concepts, it takes some time. But they can be incredibly powerful for working with data in a declarative manner.

const amountOfBlogPostsByAuthor = author => blogPostsByAuthor.get(author).length

const alexBlogPostsCount = amountOfBlogPostsByAuthor('Alex')

console.log(alexBlogPostsCount)

2

Conclusion

In this guide we learned how to utilize array.reduce(), Map and higher-order functions to create a fairly flexible method for grouping objects in an array by some key.

Do you have improvements or another neat idea? Let me know!

You can also find me on Twitter as @jvstudnitz or on my blog.