JavaScript: Flatten a nested array

JavaScript Array: Exercise-21 with Solution

Flatten Nested Array

Write a JavaScript program to flatten a nested (any depth) array. If you pass shallow, the array will only be flattened to a single level.

Sample Data:
console.log(flatten([1, [2], [3, [[4]]],[5,6]]));
[1, 2, 3, 4, 5, 6]
console.log(flatten([1, [2], [3, [[4]]],[5,6]], true));
[1, 2, 3, [[4]], 5, 6]

Sample Solution:

JavaScript Code:

// Function to flatten a nested array
var flatten = function(a, shallow, r) {
 // If the result array (r) is not provided, initialize it as an empty array
 if (!r) {
 r = [];
 }
 // If shallow is true, use concat.apply to flatten the array
 if (shallow) {
 return r.concat.apply(r, a);
 }
 // Iterate through each element in the array
 for (var i = 0; i < a.length; i++) { // Check if the current element is an array if (a[i].constructor == Array) { // Recursively flatten nested arrays flatten(a[i], shallow, r); } else { // If the current element is not an array, push it to the result array r.push(a[i]); } } // Return the flattened array return r; } // Output the result of the flatten function with a nested array console.log(flatten([1, [2], [3, [[4]]], [5, 6]])); // Output the result of the flatten function with a nested array using shallow flattening console.log(flatten([1, [2], [3, [[4]]], [5, 6]], true)); 

Output:

[1,2,3,4,5,6]
[1,2,3,[[4]],5,6]

Flowchart:

ES6 Version:

// Function to flatten a nested array
const flatten = (a, shallow, r = []) => {
 // If shallow is true, use concat and spread syntax to flatten the array
 if (shallow) {
 return [...r, ...[].concat(...a)];
 }
 // Iterate through each element in the array
 for (let i = 0; i < a.length; i++) { // Check if the current element is an array if (Array.isArray(a[i])) { // Recursively flatten nested arrays flatten(a[i], shallow, r); } else { // If the current element is not an array, push it to the result array r.push(a[i]); } } // Return the flattened array return r; }; // Output the result of the flatten function with a nested array console.log(flatten([1, [2], [3, [[4]]], [5, 6]])); // Output the result of the flatten function with a nested array using shallow flattening console.log(flatten([1, [2], [3, [[4]]], [5, 6]], true)); 

Live Demo:

See the Pen JavaScript - Flatten a nested array - array-ex- 21 by w3resource (@w3resource) on CodePen.

For more Practice: Solve these Related Problems:

• Write a JavaScript function that recursively flattens an array of arbitrary depth into a one-dimensional array.
• Write a JavaScript function that flattens an array one level deep when a shallow flag is provided.
• Write a JavaScript function that uses the reduce() method to flatten an array with both deep and shallow options.
• Write a JavaScript function that validates the input and gracefully handles non-array elements during flattening.

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