Array .flat() / flatMap()

// Flattens a nested array.
var newArray = array.flat(depth);

// Maps each element and flattens the result by one level.
var newArray = array.flatMap(function(element, index, array) {
	return transformedValue;
});

// Flatten one level of nesting.
var arr1 = [1, [2, 3], [4, 5]];
console.log(arr1.flat()); // Outputs "1,2,3,4,5".

// Flatten two levels of nesting.
var arr2 = [1, [2, [3, [4]]]];
console.log(arr2.flat(2)); // Outputs "1,2,3,[4]". Only two levels deep are flattened.

// Use Infinity to remove all levels of nesting.
console.log(arr2.flat(Infinity)); // Outputs "1,2,3,4".

// Use flatMap to produce multiple elements from a single element.
var sentences = ["Hello World", "Foo Bar"];
var words = sentences.flatMap(function(s) {
	return s.split(" ");
});
console.log(words); // Outputs "Hello,World,Foo,Bar".

// Use flatMap to filter and transform at the same time.
var numbers = [1, 2, 3, 4, 5];
var doubled = numbers.flatMap(function(n) {
	return n % 2 === 0 ? [n * 2] : [];
});
console.log(doubled); // Outputs "4,8". Only even numbers are doubled.

array.flat() is a method added in ES2019 that expands nested arrays (arrays containing other arrays) up to a specified depth into a single flat array. The original array is not modified. The default depth when the argument is omitted is 1, so only one level is flattened.

array.flatMap() combines array.map() and array.flat(1) into a single method. When the result of transforming each element is an array, it is automatically flattened by one level. This makes it easy to generate multiple elements from a single element, or to exclude elements by returning an empty array.

array.flat() also removes empty slots. For example, [1, , 3].flat() returns [1, 3], removing any holes in the array. To flatten all levels of nesting, use array.flat(Infinity).