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Lodash _.flatMapDeep() Method - GeeksforGeeks

Last Updated : 15 Jul, 2025

Lodash _.flatMapDeep() method creates a flattened array of values by running each element in the given collection through the iteratee function and recursively flattens the mapped results. It is similar to the _.flatMap() method.

_.flatMapDeep(collection, iteratee);

• collection: It is the collection to iterate over.
• iteratee: It is the function that is invoked per iteration.

This method returns the new flattened array.

Example 1: In this example, we are using the _.flatMapDeep() method for duplicating the elements of a given array.

// Requiring the lodash library 
const _ = require("lodash");

// Original array 
let users = (['aaa', 'bbb', 'ccc',
    'ddd', 'eee', 'fff']);

// Using the _.flatMapDeep() method
let flat_map =
    _.flatMapDeep(users,
        function duplicate(n) {
            return [[[n, n]]];
        }
    )

// Printing the output 
console.log(flat_map);

Output:

[
  'aaa', 'aaa', 'bbb', 'bbb',
  'ccc', 'ccc', 'ddd', 'ddd',
  'eee', 'eee'
]

Example 2: In this example, we are using the _.flatMapDeep() method for naking apatter of a given array.

// Requiring the lodash library 
const _ = require("lodash");

// Original array 
let user1 = ([1, 2, 3, 4, 5, 6, 7]);
let user2 = (['a', 'b', 'c', 'd', 'e']);

// Using the _.flatMapDeep() method
let flat_map1 =
    _.flatMapDeep(user1,
        function makePattern(n) {
            return [[[n, n + ">"]]];
        }
    )

let flat_map2 =
    _.flatMapDeep(user2,
        function makePattern(n) {
            return [[["<" + n, n]]];
        }
    )

// Printing the output 
console.log(flat_map1);
console.log(flat_map2);

Output:

[
  1, '1>', 2, '2>',
  3, '3>', 4, '4>',
  5, '5>', 6, '6>',
  7, '7>'
]
[
  '<a', 'a',  '<b',
  'b',  '<c', 'c',
  '<d', 'd',  '<e',
  'e'
]

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