Here are the features introduced in ES10 and ES11 :

ECMAScript 2019 (ES10):

1. Array.prototype.flat(): Flattens nested arrays to a specified depth.

const nestedArray = [1, [2, [3, [4]]]];
const flattenedArray = nestedArray.flat(2);
console.log(flattenedArray); // Output: [1, 2, 3, [4]]

2. Array.prototype.flatMap(): Maps and flattens array elements.

const arr = [1, 2, 3];
const mappedAndFlattened = arr.flatMap(x => [x * 2]);
console.log(mappedAndFlattened); // Output: [2, 4, 6]

3. String.prototype.trimStart(): Removes leading whitespace from a string.

const str = "   Hello, world!   ";
const trimmedStr = str.trimStart();
console.log(trimmedStr); // Output: "Hello, world!   "

4. String.prototype.trimEnd(): Removes trailing whitespace from a string.

const str = "   Hello, world!   ";
const trimmedStr = str.trimEnd();
console.log(trimmedStr); // Output: "   Hello, world!"

5. Function.prototype.toString(): Allows access to function source code.

function greet() {
  return "Hello!";
}
const greetSource = greet.toString();
console.log(greetSource); // Output: "function greet() { return \"Hello!\"; }"

6. Object.fromEntries(): Creates an object from key-value pairs.

const entries = [['a', 1], ['b', 2]];
const obj = Object.fromEntries(entries);
console.log(obj); // Output: { a: 1, b: 2 }

7. Symbol.prototype.description: Provides a description for symbols.

const mySymbol = Symbol('My Symbol');
console.log(mySymbol.description); // Output: "My Symbol"

8. try...catch with optional binding: Simplified error handling with optional catch parameter.

try {
  throw new Error('Oops!');
} catch {
  console.log('Error caught!');
}
// Output: "Error caught!"

ECMAScript 2020 (ES11):

1. BigInt: Data type for arbitrary precision integers.

const bigIntValue = 9007199254740991n;
console.log(bigIntValue); // Output: 9007199254740991n

2. Optional Chaining: Simplifies accessing nested properties.

const user = {
  name: 'John',
  address: {
    city: 'New York'
  }
};
const city = user.address?.city;
console.log(city); // Output: "New York"

3. Nullish Coalescing Operator: Provides a default value for nullish values.

const value1 = null;
const value2 = "Hello, world!";
const result = value1 ?? value2;
console.log(result); // Output: "Hello, world!"

4. Promise.allSettled(): Handles multiple promises, regardless of rejection or fulfillment.

const promises = [
  fetch('/data'),
  fetch('/users'),
  fetch('/settings')
];
Promise.allSettled(promises).then(results => console.log(results));

5. String.prototype.matchAll(): Returns an iterator of all regex matches.

const regex = /\d+/g;
const str = '10 apples and 25 oranges';
const matches = [...str.matchAll(regex)];
console.log(matches);

6. globalThis: A reference to the global this value.

console.log(globalThis === window); // Output: true (in a browser environment)

7. import(): Dynamically imports modules.

const modulePath = './my-module.js';
import(modulePath).then(module => console.log(module));

Please refer to the official ECMAScript website (tc39.es) or the ECMAScript proposal repository (github.com/tc39/proposals).