In this third article of the Array method series, I'll explain the flat method and how it works.

Here's the video version: Array.flat() YouTube

What is the Flat Method?

The flat method of arrays is used to flatten nested arrays within an array. What are nested arrays?

const array = [1, 2, [3, 4, [5, 6]]]

[3, 4, [5, 6]] is nested within the main array, and [5,6] is also nested in the previous array.

With the flat method, you can concatenate these nested arrays to the parent array.

The returned output of the flat method is a new array containing the concatenated items.

Syntax of the Flat method

array.flat(depth)

The depth argument specifies the nest level you want to concatenate. Using this example:

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

Depth 1 - [3, 4, [5, 6]
Depth 2 - [5, 6]
Depth 3 - There's no third depth

You can also pass a depth of Infinity, which goes deeper to unnest the deepest nested array 🔥 haha.

Without the Flat method

Here's how to flatten an array without the Flat method:

const array = [1, 2, [3, 4, [5, 6]]]
const flattenedArr = []

// for 2 levels
for (let i = 0; i < array.length; i++) {
  const item = array[i]

  if (!Array.isArray(item)) {
    // not an array
    flattenedArr.push(item)
  } else {
    // for the first level
    for (let j = 0; j < item.length; j++) {
      const item2 = item[j]

      if (!Array.isArray(item2)) {
        // not an array
        flattenedArr.push(item2)
      } else {
        // for the second level
        flattenedArr.push(...item2)
      }
    }
  }
}

console.log(flattenedArr)
// [ 1, 2, 3, 4, 5, 6 ]

Looping and looping, just to flatten an array. The flat method is definitely a lifesaver 😅

NB: This is not the only way to do it without the flat method. Check out more ways in this StackOverflow discussion

With the Flat method

const array = [1, 2, [3, 4, [5, 6]]]
const flattenedArr = array.flat(2)
// or array.flat(Infinity)

console.log(flattenedArr)
// [ 1, 2, 3, 4, 5, 6 ]

Browser Support

The flat method was introduced in ES2019 and currently, the browser support gotten from MDN is displayed here:

Pretty good.