Lodash _.flattenDeep() and _.flattenDepth() Method

20 June 2025

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Lodash _.flattenDeep() Method

The _.flattenDeep() method is used to completely flatten nested arrays. It does this recursively.

Syntax:

_.flattenDeep( array )

Parameters: This method accepts single parameter as mentioned above and described below:

Return Value: This method returns the new flattened array.

Example:

const _ = require('lodash'); 
  
let ar = [1, [2, [3, 4, [5]]]]; 
  
let flattenArray = _.flattenDeep(ar); 
  
console.log(flattenArray);

Here, const _ = require('lodash') is used to import the lodash library into the file.

Output:

[ 1, 2, 3, 4, 5 ]

Lodash _.flattenDepth() Method

The _.flattenDepth() method is used to flatten up to depth time that is passed into the function.

Syntax:

_.flattenDepth(array, depth)

Parameters: This method accepts two parameters as mentioned above and described below:

Return Value: This method returns the new flattened array.

Example 1: Flatten to depth 1

const _ = require('lodash'); 
  
let ar = [1, [2, [3, 4, [5]]]]; 
  
let flattenArray = _.flattenDepth(ar, 1); 
  
console.log(flattenArray);

Output:

[ 1, 2, [ 3, 4, [ 5 ] ] ]

Example 2: Flatten to depth 2

const _ = require('lodash'); 
  
let ar = [1, [2, [3, 4, [5]]]]; 
  
let flattenArray = _.flattenDepth(ar, 2); 
  
console.log(flattenArray);

Output:

[ 1, 2, 3, 4, [ 5 ] ]

Note: This will not work in normal JavaScript because it requires the library lodash to be installed.

Reference: