ECMAScript

2020 Highlights

dynamic imports

This enables deciding what to import at runtime.

import('/some/name.js').then(module => {
  // use the module
});

const module = await import('/some/name.js');

namespace re-exporting

export {foo, bar as baz} from 'some-module';
export _ from 'some-module';
export _ as newName from 'some-module';

optional chaining

const zip = person?.address?.zipCode;
// sets to undefined if person or person.address is undefined

const bloodType = person?.getHealthData?.().getBloodType?.();
// sets to undefined if person has no getHealthData method or if
// the object returned by getHealthData has no getBloodType method

nullish coalescing

const age = person.age || 21;
// sets to 21 if expression on left
// evaluates to a falsey value, including zero

const age = person.age ?? 21;
// sets to 21 if expression on left
// evaluates to undefined or null

BigInt type

supports arbitrarily large integer values

String matchAll method

returns iterator over all matches of a regular expression that must have “g” flag

const someText = 'fooA1barB2bazC3';
const matches = someText.matchAll(/[A-Z][0-9]/g);
for (const match of matches) {
  console.log(match[0], 'begins at', match.index);
}

outputs: A1 begins at 3 B2 begins at 8 C3 begins at 13

Promise.allSettled
- takes an Iterable of promises
- resolves after all either resolve or reject
- resolved value is an array of object that each have a status property
- if status is 'fulfilled', a value property is also present
- if status is 'rejected', a reason property is also present
globalThis
- provides a standard way to access the “global object” in all environments
- alternative to window, self, or frames on the web
- alternative to self in Web Workers
- alternative to global in Node.js

2021 Highlights

numeric separators for readability (underscore)

ex. 1_234_567.89

String replaceAll method

s.replace(/foo/g, 'bar')
can be written as
s.replaceAll('foo', 'bar')

Promise.any
- takes an iterable of promises
- resolves with the first one resolves
- if all reject, rejects with an AggregateError exception that holds an error from each promise in order
- alternative to Promise.race that resolves or rejects based on the first promise to complete
private class members
- begin name of fields and methods with #
- applies to both instance and static members

2022 Highlights

expected to be finalized in June 2022

can declare public fields of a class (instance or static) inside class, not in constructor

class Dog {
  static count = 0;
  breed = '';

  constructor(breed) {
    this.breed = breed;
    Dog.count++;
  }
}

static blocks
- used to initialize static fields in a class when their values must be computed
- syntax: static { code }
- can have more than one in the same class
top-level await

can use await keyword outside of async functions
error cause

Error objects can have a cause property that refers to another Error object
at method added to Array and String
- takes an index which can be negative to count from the end
- myArr.at(0) returns the first element
- myArr.at(-1) returns the last element
RegExp match indices

Adding the /d flag to a regular expression causes it to record the start and end index of each capture group.