Array.prototype.flat()

let arr = [1, 2, 3, [4, 5, 6, [7, 8, 9, [10, 11, 12]]]];

arr.flat();
// [1, 2, 3, 4, 5, 6, [7, 8, 9, [10, 11, 12]]]
arr.flat().flat();
// [1, 2, 3, 4, 5, 6, 7, 8, 9, [10, 11, 12]]
arr.flat(3);
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
// Or, if you're not sure about the depth of the array:
arr.flat(Infinity);
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

Reference: MDN web docs - Array.prototype.flat()

Array.prototype.flatMap()

let arr = [1, 2, 3, 4];

arr.map(x => [x, x * 2]);
// [[1, 2], [2, 4], [3, 6], [4, 8]]

arr.flatMap(x => [x, x * 2]);
// [1, 2, 2, 4, 3, 6, 4, 8]

Reference: MDN web docs - Array.prototype.flatMap()

String.prototype.trimStart() and String.prototype.trimEnd()

const greeting = '   Hello world!   ';

console.log(`"${greeting.trimStart()}"`);
// expected output: "   Hello world!   ";

console.log(`"${greeting.trimStart()}"`);
// expected output: "Hello world!   ";

console.log(`"${greeting.trimEnd()}"`);
// expected output: "   Hello world!";

References:

MDN web docs - String.prototype.trimStart()\

MDN web docs - String.prototype.trimEnd()

Optional Catch Binding

Before ES2019

try {
  // do something that raises exception
} catch (err) {
  // err is required even if it is not being used in catch block
}

With ES2019

try {
  // do something that raises exception
} catch {
  // no binding required if it is not being used
}

Object.fromEntries()

// Iterable object such as Array or Map
const entries = [
  ['foo', 'bar'],
  ['baz', 42]
];

const obj = Object.fromEntries(entries);

console.log(obj);
// expected output: Object { foo: "bar", baz: 42 }

Reference: MDN web docs - Object.fromEntries()