Overview of ES2020 New JavaScript Features

ES2020 introduced several new JavaScript features that improve asynchronous handling, global object access, optional chaining, nullish coalescing, module metadata, big integers, dynamic imports, private class fields, matchAll, deterministic for‑in order, and module namespace exports.

#1 Promise.allSettled returns a promise that resolves after all input promises settle, providing an array of objects with status , value (if fulfilled) or reason (if rejected). Example:

const sleep = (timeout) => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve("sleep finish!");
    }, timeout);
  });
};

Promise.allSettled([sleep(2000), Promise.reject(new Error("error"))])
  .then(res => {
    console.log("res", res);
  });

#2 globalThis offers a standard way to access the global object across environments, e.g., globalThis.location === window.location; // true .

#3 Optional Chaining (?.) safely accesses deep properties without explicit checks. Example:

const adventurer = { name: 'Alice', cat: { name: 'Dinah' } };
console.log(adventurer.dog?.name); // undefined

#4 Nullish Coalescing (??) returns the right‑hand operand when the left is null or undefined , otherwise the left operand.

#5 import.meta provides module metadata; in browsers it contains the module URL. Example:

console.log(import.meta); // {url: "http://127.0.0.1:8099/html/2020-JavaScript/index.js"}

#6 BigInt enables representation of integers larger than 2^53‑1 . Literal syntax appends n , e.g., 9007199254740991n . BigInt is not strictly equal to Number but loosely equal.

#7 Dynamic Import allows modules to be loaded on demand, returning a promise. Example:

if (flag) {
  import('./module.js').then(fn => {
    fn.say();
  });
}

#8 Private Class Variables use # prefix for private fields, methods, and static members, which are only accessible within the class body.

class Cat {
  #name = 'tom';
  getName() {
    console.log(this.#name);
  }
}

#9 String.prototype.matchAll returns an iterator of all matches, including capture groups. Example:

let regexp = /t(e)(st(\d?))/g;
let str = 'test1test2';
let array = [...str.matchAll(regexp)];
console.log(array[0]); // ["test1", "e", "st1", "1"]

#10 for…in order is now standardized in ES2020, guaranteeing a deterministic property enumeration order.

#11 Module namespace exports allow re‑exporting all exports under a single name, e.g., export * as utils from './module.js'; .