Here are the key features introduced in ES9 (ECMAScript 2018), with detailed descriptions and examples:

1. Rest/Spread Properties:

• Rest Property: Gathers remaining object properties into a new object, for flexible object manipulation.

JavaScript

const { name, ...otherProps } = person; // name gets the "name" property, otherProps gets the rest

• Spread Property: Spreads properties from one object into another, for easy object copying and merging.

JavaScript

const newPerson = { ...person, age: 35 }; // Copies person's properties and adds an "age" property

2. Asynchronous Iteration:

• for-await-of: Iterates over asynchronous iterables (like data arriving over time), handling each element as it becomes available.

JavaScript

async function fetchUserData() {
  const response = await fetch('/users');
  const users = await response.json();
  for await (const user of users) {
    console.log(user);
  }
}

3. Promise.prototype.finally():

• Executes a callback regardless of whether a Promise is resolved or rejected, ensuring cleanup actions or logging.

JavaScript

fetchData()
  .then(data => console.log(data))
  .catch(error => console.error(error))
  .finally(() => console.log('I always run!'));
Use code with caution. Learn more

4. RegExp Named Capture Groups:

• Assign names to capture groups in regular expressions for easier extraction and readability.

JavaScript

const match = /(?<name>[A-Z][a-z]+) (?<age>\d+)/.exec("Alice 30");
console.log(match.groups); // { name: "Alice", age: "30" }

5. RegExp Lookbehind Assertions:

• Assert patterns behind a match without including those patterns in the match result, enabling more specific matching rules.

JavaScript

const match = /(?<=abc)def/.exec("abcdef"); // Matches "def" only if preceded by "abc"

6. RegExp dotAll Flag (s):

• Makes the dot (.) match newline characters, allowing for broader pattern matching across multiple lines.

JavaScript

const match = /a.b/s.test("a\nb"); // true with dotAll flag

7. Object.fromEntries():

• Creates an object from a list of key-value pairs, often used with Object.entries() for object manipulation.

JavaScript

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