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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