Flatten an Array in JavaScript: Use flat() to Handle Nested Arrays

Arrays often hold other arrays. This happens with API responses, form data, or nested objects. These layers add extra steps when you want to work with the values. A flat array in JavaScript removes the inner arrays and gives you one simple list.

Let’s move on to the following section to cover everything you need to learn about how the flat() function works in JavaScript and see examples.

What Does It Mean to Flat an Array in JavaScript?

JavaScript includes a built-in method called .flat() to flatten arrays. It allows you to remove inner arrays and combine all values into one list.

Syntax:

array.flat([depth])

• array is the original array.
• depth is optional. It tells how deep to flatten the array.
• The default depth is 1.

Parameters:

• flat(1) flattens one level (default).
• flat(2) flattens two levels.
• flat(Infinity) flattens all nested levels.

For example:

const nested = [1, [2, 3], [4, [5]]];
const flat = nested.flat(2); 

Output:

 [1, 2, 3, 4, 5]

The nested array holds numbers and other arrays inside it. The flat(2) call removes two levels of nesting and returns one flat list.

Understand the Deep Flat in JavaScript

A deep flat means you want to flatten an array all the way, no matter how many nested levels it has.

To do this, pass Infinity as the depth:

const arr = [1, [2, [3, [4, 5]]]];
const deepFlat = arr.flat(Infinity);

The output:

[1, 2, 3, 4, 5]

So in this context:

• flat() = shallow flatten (default is 1 level)
• flat(Infinity) = deep flatten (removes all nested levels)

Use Infinity when you don’t know how many levels the nesting goes.

Browser Support and Polyfill for flat()

The Array.prototype.flat() method is supported in most modern browsers:

This code adds support for .flat() in older browsers:

if (!Array.prototype.flat) {
  Array.prototype.flat = function(depth = 1) {
    return this.reduce((acc, val) => {
      if (Array.isArray(val) && depth > 0) {
        acc.push(...val.flat(depth - 1));
      } else {
        acc.push(val);
      }
      return acc;
    }, []);
  };
}

console.log([12,3,[14, [undefined, 15, [null, true]]],55,[5544]].flat(5));

The output:

[ 12, 3, 14, undefined, 15, null, true, 55, 5544 ]

It checks if .flat() exists. If not, it defines the method using reduce and recursion. Add this before you use .flat() to make sure the method works everywhere.

Examples

Nested arrays:

You can flatten nested arrays with the flat function. It works in modern browsers and takes an optional depth value.

const numbers = [10, [20, 30], [40, [50]]];
const firstLevel = numbers.flat(1);
console.log(firstLevel);  

Output:

[ 10, 20, 30, 40, [ 50 ] ]

This removes only the first layer of nesting. Deeper arrays stay as they are.

Flatten all levels:

const deepArray = [10, [20, [30, [40]]]];
const fullFlat = deepArray.flat(Infinity);
console.log(fullFlat);  

Output:

[10, 20, 30, 40]

You can pass Infinity to flatten every layer, no matter how deep.

Flatten an array with custom objects:

const data = [
  { id: 1 },
  [{ id: 2 }, { id: 3 }],
  [[{ id: 4 }], [{ id: 5 }]],
];

const flatData = data.flat(2);
console.log(flatData);

Here you have a list of objects nested in arrays. The .flat(2) call brings all objects to one level, so you can loop through them. It doesn’t need a check for depth.

Flatten deeply nested form data:

const formData = [
  ['name', 'John'],
  [['email', 'coder@example.com']],
  [[[['age', 30]]]],
];

const cleanData = formData.flat(Infinity);
console.log(cleanData);

Output:

[ 'name', 'John', 'email', 'coder@example.com', 'age', 30 ]

If form data comes in layers from a server or script, you can flatten it completely. This gives a clean list you can loop through or pair up later.

Wrapping Up

You learned what it means to flatten an array and why it’s useful for nested data. You also explored how to use the built-in .flat() method and how the depth parameter works.

Here’s a quick recap:

• Use .flat() to turn nested arrays into a single-level array.
• Set depth to control how many levels to flatten.
• Use Infinity for full flattening.
• Add a polyfill if you need support in older browsers.

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