JavaScript: Flatten a nested array

Last update on May 30 2022 10:07:13 (UTC/GMT +8 hours)

JavaScript Array: Exercise-21 with Solution

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

HTML Code:

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Flatten a nested array</title>
</head>
<body>
</body>
</html>

JavaScript Code:

var flatten = function(a, shallow,r){
  if(!r){ r = []}
   
if (shallow) {
  return r.concat.apply(r,a);
  }
      
   for(var i=0; i<a.length; i++){
        if(a[i].constructor == Array){
            flatten(a[i],shallow,r);
        }else{
            r.push(a[i]);
        }
    }
    return r;
}

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

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

Sample Output:

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

Flowchart:

ES6 Version:

const flatten = (a, shallow, r) => {
  if(!r){ r = []}
   
if (shallow) {
  return r.concat(...a);
  }
      
   for(let i=0; i<a.length; i++){
        if(a[i].constructor == Array){
            flatten(a[i],shallow,r);
        }else{
            r.push(a[i]);
        }
    }
    return r;
};

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

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.

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Next: Write a JavaScript program to compute the union of two arrays.

JavaScript: Tips of the Day

How to insert an item into an array at a specific index (JavaScript)?

What you want is the splice function on the native array object.

arr.splice(index, 0, item); will insert item into arr at the specified index (deleting 0 items first, that is, it's just an insert). In this example we will create an array and add an element to it into index 2:

var arr = []; arr[0] = "Jani"; arr[1] = "Hege"; arr[2] = "Stale"; arr[3] = "Kai Jim"; arr[4] = "Borge"; console.log(arr.join()); arr.splice(2, 0, "Lene"); console.log(arr.join());

Ref: https://bit.ly/2BXbp04