The flatMap() array method in JavaScript

Published July 29, 2020

The flatMap() is an array method and is used to do some operation before converting a multidimementional array into a single array.

Suppose we have an array of sentences like this,

// an array of sentences
let arr = [
  "Hello, How are you?",
  "Harry is a good man",
  "Hey, John",
  "I'm Fine",
];

What if we want to split all the words by spaces?

We can do that using the map() and split() methods like this.

// an array of sentences
let arr = [
  "Hello, How are you?",
  "Harry is a good man",
  "Hey, John",
  "I'm Fine",
];

// loop through every elements using map
// and then split each array element by space
let newArr = arr.map((element) => {
  return element.split(" ");
});

console.log(newArr);

Now if we look into the array, we can see something like this,

[
    ["Hello,", "How", "are", "you?"],
    ["Harry", "is", "a", "good", "man"],
    ["Hey,", "John"],
    ["I'am", "Fine"]
]

Here we have all the words in a nested array. But we never wanted our words to be in a nested array. The split() method inside the map() method always returns every element inside a nested array, That's a problem for us.

That's where the flatMap() method comes to help us.

The flatMap() method first maps through every element in an array and then converts every nested array elements into a single non-dimensional array.

Let's try what we did before with the flatMap() array method.

// an array of sentences
let arr = [
  "Hello, How are you?",
  "Harry is a good man",
  "Hey, John",
  "I'm Fine",
];

// flatMap() method first maps through every element
// and then converts nested array into single array
let newArr = arr.flatMap((element) => {
  return element.split(" ");
});

console.log(newArr);

This is the output,

// everything in a single array
[
    "Hello,",
     "How",
     "are",
     "you?",
     "Harry",
     "is",
    "a",
    "good",
    "man",
    "Hey,",
    "John",
    "I'am",
    "Fine"
]

The flatMap() method accepts a callback function and passes the current element which is getting looped.

This code is more elegant and readable than our previous code.

The flatMap() does the job of mapping through the array first and then converts a nested array into a single array.

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