JavaScript Array flatMap () Definition:

The flatMap () method maps each value to a new value, and then returns the resulting array to a maximum depth of 1. To be more specific, for example, a series called [1, 2]. Let’s set this sequence to a function that will multiply each element by 2. Let’s apply the flatMap () method to the function that we match. This is our new series: [2, 4].

Let’s see if we can encode the above example?

        let list = [1, 2]
        let arr  = list.flatMap(x =>[x * 2])
        console.log(arr)

[ 2 , 4 ]

Another advantage of using this method is that it shortens the codes. This method is a combination of the flat () and map () methods. This way we will do the same work with less code.

flat(): The flat() method available on the Array prototype returns a new array that’s a flattened version of the array it was called on. For more information about flat () please click here.

JavaScript Array flatMap() Syntax:

var new_array = arr.flatMap(function callback(currentValue[, index[, array]]) {
    // return element for new_array
}[, thisArg])

callback:
Function that produces an element of the new Array, taking three arguments:
currentValue:
The current element being processed in the array.
indexOptional:
The index of the current element being processed in the array.
arrayOptional:
The array map was called upon.
thisArgOptional:
Value to use as this when executing callback.

JavaScript Array flatMap() Examples

Example 1

        let list = [1, 2, 3, 4, 5]
        let result = list.flatMap(x =>[x * 10]);
        console.log(result)

output:

[10, 20, 30, 40, 50]

Example 2

        let list = [1, 2, 3, 4, 5]
        let result = list.flatMap(x =>[[x * 10]]);
        console.log(result)

output:

[Array(1), Array(1), Array(1), Array(1), Array(1)]
0: [10]
1: [20]
2: [30]
3: [40]
4: [50]

Example 3

        let list1 = ["john", "barry", "oliver"];
        let list2 = ["rihanna", "jessica", "lisa"];

        let combine = list1.flatMap((name, index) =>[name, list2[index]]);

        console.log(combine)

output:

[“john”, “rihanna”, “barry”, “jessica”, “oliver”, “lisa”]

Example 4

        let arr = ["javascript", "flatmap()"];

        console.log(arr.flatMap(x => x.split("")));
        console.log(arr.flatMap(x => x.split(" ")));

output:

[“j”, “a”, “v”, “a”, “s”, “c”, “r”, “i”, “p”, “t”, “f”, “l”, “a”, “t”, “m”, “a”, “p”, “(“, “)”]
[“javascript”, “flatmap()”]

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