const arr = [1, 2, [3, 4, [5, 6]]];

console.log(arr.flat()); // [1, 2, 3, 4, [5, 6]]
console.log(arr.flat(2)); // [1, 2, 3, 4, 5, 6]
console.log(arr.flat(Infinity)); // [1, 2, 3, 4, 5, 6]

Array.prototype.flatMap

Maps each element using a mapping function, then flattens the result by one level.

const sentences = ["Hello world", "Good morning"];
const words = sentences.flatMap(sentence => sentence.split(" "));
console.log(words); // ["Hello", "world", "Good", "morning"]

// Equivalent to map().flat()
const words2 = sentences.map(sentence => sentence.split(" ")).flat();

Object.fromEntries

Transforms a list of key-value pairs into an object.

const entries = [["name", "John"], ["age", 30]];
const obj = Object.fromEntries(entries);
console.log(obj); // { name: "John", age: 30 }

// With Map
const map = new Map([["a", 1], ["b", 2]]);
const objFromMap = Object.fromEntries(map);
console.log(objFromMap); // { a: 1, b: 2 }