depth Optional

The depth level specifying how deep a nested array structure should be flattened. Defaults to 1.

A new array with the sub-array elements concatenated into it.

The flat() method is a copying method. It does not alter this but instead returns a shallow copy that contains the same elements as the ones from the original array.

The flat() method ignores empty slots if the array being flattened is sparse. For example, if depth is 1, both empty slots in the root array and in the first level of nested arrays are ignored, but empty slots in further nested arrays are preserved with the arrays themselves.

The flat() method is generic. It only expects the this value to have a length property and integer-keyed properties. However, its elements must be arrays if they are to be flattened.

The flat() method removes empty slots in arrays:

const arr5 = [1, 2, , 4, 5];
console.log(arr5.flat()); // [1, 2, 4, 5]

const array = [1, , 3, ["a", , "c"]];
console.log(array.flat()); // [ 1, 3, "a", "c" ]

const array2 = [1, , 3, ["a", , ["d", , "e"]]];
console.log(array2.flat()); // [ 1, 3, "a", ["d", empty, "e"] ]
console.log(array2.flat(2)); // [ 1, 3, "a", "d", "e"]

The flat() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length. If the element is not an array, it's directly appended to the result. If the element is an array, it's flattened according to the depth parameter.

const arrayLike = {
  length: 3,
  0: [1, 2],
  // Array-like objects aren't flattened
  1: { length: 2, 0: 3, 1: 4 },
  2: 5,
  3: 3, // ignored by flat() since length is 3
};
console.log(Array.prototype.flat.call(arrayLike));
// [ 1, 2, { '0': 3, '1': 4, length: 2 }, 5 ]

• Polyfill of Array.prototype.flat in core-js
• Indexed collections guide
• Array
• Array.prototype.concat()
• Array.prototype.flatMap()
• Array.prototype.map()
• Array.prototype.reduce()