ES9

ECMAScript 9, commonly referred to as ECMAScript 2018, is the ninth edition of the ECMAScript language specification. Published in 2018 by Ecma International, it follows ES8 (2017) and introduces a set of practical features aimed at simplifying common JavaScript tasks, including object manipulation, asynchronous programming, and enhanced regular expressions.

Object rest/spread properties were added to simplify object composition and data extraction. Rest properties collect the remaining properties of an object into a new object, while spread properties copy properties from one object into another. Example: const { a, ...rest } = obj; const newObj = { ...obj, b: 2 }.

Asynchronous iteration introduces the for-await-of loop and the AsyncIterable protocol, enabling native-style iteration over data sources that deliver values asynchronously, such as streams. Example: for await (const chunk of stream) { … }.

Promise.prototype.finally provides a cleanup callback that runs when a promise settles, regardless of its outcome, allowing cleanup logic without duplicating code in both success and error paths.

RegExp improvements include lookbehind assertions, which allow patterns to reference preceding text without including it in the match; named capture groups that assign names to captured substrings; Unicode property escapes to match characters by their Unicode properties; and the dotAll (s) flag so the dot matches newline characters.

Support for ES9 features varies by platform, but modern JavaScript engines and environments broadly implement these features, contributing to more expressive code and streamlined asynchronous patterns in contemporary web and server applications.