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Amit Khonde

What is flatMap in JavaScript? And how made my code cleaner

Have you ever looked at a JavaScript in built function and said to yourself:

I understand what this is but I do not know where I can use this.

Well, this exact thought came to mind when I read about Array.flatMap(). So I started looking for its applications in my day-to-day coding and viola!! I found a use case. In this post, we will first see what Array.flatmap() is and then look at how I used it in a real problem.

What exactly is Array.flatMap?

FlatMap is a mix of two words which are derived from latin words blah blah blah...

Just kidding.

Array.flatMap is a mixture of two array methods Array.flat and Array.map. Individually,

  • Array.flat is used for flattening nested arrays. For example
1 2 3 4 const names = ['Amit', 'Karishma', 'Kunal', ['Pankaj', 'Rahee']]; console.log(names.flat()); // expected output: ["Amit", "Karishma", "Kunal", "Pankaj", "Rahee"]
  • Array.map is used for creating a new array by performing operations on each array element and returning a value. For example
1 2 3 4 5 6 7 const names = ['Amit', 'Karishma', 'Kunal', 'Pankaj', 'Rahee']; const everyOneLovesDogs = names.map((name) => { return `${name} loves dogs`; }); console.log(everyOneLovesDogs); // expected output: ["Amit loves dogs", "Karishma loves dogs", "Kunal loves dogs", "Pankaj loves dogs", "Rahee loves dogs"]

So as you can guess, Array.flatMap maps an array and then flattens the nested arrays in the returned array. A quick example of flatMap looks like this:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 const usersWithInterests = [ { name: 'Amit', interests: ['Cricket', 'Music'], }, { name: 'Karishma', interests: ['Drawing', 'Reading'], }, { name: 'Pranav', interests: ['Crafting', 'Biking'], } ]; const interests = usersWithInterests.flatMap((user) => { return user.interests; }); console.log(interests); // Expected Output: ["Cricket", "Music", "Drawing", "Reading", "Crafting", "Biking"]

How I used Array.flatMap in a real-world problem?

I was working on a feature where I had to create a dropdown for categories for products. The actual response from getProducts API looks like this:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 const response = { "status": "success", "products": [ { "name": 'One Plus 7T', "categories": ['Technology', 'Mobile', 'SmartPhone'], "description": 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.', "rating": '4.5' }, { "name": 'MacBook Pro 2018', "categories": ['Technology', 'Computer', 'Laptop'], "description": 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.', "rating": '4.8' }, { "name": 'LG Monitor 221G', "categories": ['Technology', 'Monitor'], "description": 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.', "rating": '4.3' } ] }

The product object has categories as a key for which the values are stored in the array.

So I had to do 3 things

  • Get the categories from each product
  • Flatten that array
  • Get unique values from the array.

Instead of using 2 separate functions for first two tasks, i used one function: Array.flatmap. And then applied the set operation on that array to get unique values.

The final code snippet looked something like this:

1 2 3 4 5 6 7 8 9 const { products } = response; const allCategories = products.flatMap((product) => { return product.categories; }); const uniqueCategories = [...new Set(allCategories)]; console.log(uniqueCategories); // Expected Output: ["Technology", "Mobile", "SmartPhone", "Computer", "Laptop", "Monitor"]


I hope now you folks can also apply Array.flatMap in cases where they fit and produce a cleaner code. For more such interesting tidbits, stay tuned in. Until then, Happy Coding!!


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