Last Updated : 15 Jul, 2025
The Javascript arr.flat() method was introduced in ES2019. The flat() method in JavaScript creates a new array with all sub-array elements concatenated into it recursively up to the specified depth. If no depth is provided, it defaults to 1.
arr.flat([depth])Parameters:
This method accepts a single parameter as mentioned above and described below:
It returns an array i.e. depth levels flatter than the original array, it removes nesting according to the depth levels.
Example 1: Flattening a Multilevel Array
The code initializes a multilevel array, then applies the flat() method with Infinity parameter to recursively flatten all nested arrays into a single-level array. The result is logged.
JavaScript
// Creating multilevel array
const numbers = [['1', '2'], ['3', '4',
['5', ['6'], '7']]];
const flatNumbers = numbers.flat(Infinity);
console.log(flatNumbers);
[ '1', '2', '3', '4', '5', '6', '7' ]
Example 2: Flattening Nested Arrays with flat() Method
The code demonstrates flattening a nested array to different levels using the flat() method. It applies flat() with various depth parameters (0, 1, 2) to flatten nested arrays accordingly and logs the results.
JavaScript
let nestedArray = [1, [2, 3], [[]],
[4, [5]], 6];
let zeroFlat = nestedArray.flat(0);
console.log(
`Zero levels flattened array: ${zeroFlat}`);
// 1 is the default value even
// if no parameters are passed
let oneFlat = nestedArray.flat(1);
console.log(
`One level flattened array: ${oneFlat}`);
let twoFlat = nestedArray.flat(2);
console.log(
`Two level flattened array: ${twoFlat}`);
// No effect when depth is 3 or
// more since array is already
// flattened completely.
let threeFlat = nestedArray.flat(3);
console.log(
`Three levels flattened array: ${threeFlat}`);
Zero levels flattened array: 1,2,3,,4,5,6 One level flattened array: 1,2,3,,4,5,6 Two level flattened array: 1,2,3,4,5,6 Three levels flattened array: 1,2,3,4,5,6
Note: For depth greater than 2, the array remains the same, since it is already flattened completely.
Example 3: Flattening an Array with undefined Element
This code creates an array arr with elements [1, 2, 3, empty, 4]. When flat() is called on arr, it removes the empty slot and creates a new flattened array newArr with elements [1, 2, 3, 4], which is then logged to the console.
let arr = [1, 2, 3, , 4];
let newArr = arr.flat();
console.log(newArr);
We have a complete list of Javascript Array methods, to check those please go through this Javascript Array Complete reference article.
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