September 15th, 2021 × #JavaScript#ECMAScript#ES2022

New to JavaScript — ES2022

Overview of new JavaScript language features added in ECMAScript 2022 including top level await, the array .at() method, private fields and methods in classes, regex indices and more.


In this episode of Syntax, Scott and Wes talk about all the new stuff in ES2022 — what it is, why you might need it, and how to use it.

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Show Notes

04:50 - Regex indicies

  • New d flag in a regex
  • https://regex101.com/
  • This will tell you the indexes (indicies) of the regex matches
  • Handy if you need to highlight or replaces matches in a string
  • We can ask for the start and end positions of each matched capture group

07:16 - Class updates

  • Private fields
  • Properties and Methods to be kept private
  • Prefix them with a #
  • =Helpful for internal state and methods which should not be accessed directly or at all by external
  • In React how we have __INTERNTAL_NEVER USE THIS

class ColorButton extends HTMLElement {
  // All fields are public by default
  color = "red"

  // Private fields start with a #, can only be changed from inside the class
  #clicked = false

const button = new ColorButton()
// Public fields can be accessed and changed by anyone
button.color = "blue"

// SyntaxError here 
console.log(button.#clicked) // Cannot be read from outside
button.#clicked = true // Cannot be assigned a value from outside
class Person {
  #hobbies = ['computers']
  get #hobbiesGetter() { return this.#hobbies }
	#getHobbies() { return this.#hobbies }
	getHobbiesPublic() { return this.#hobbies }

const scott = new Person();

scott.#getHobbies(); // doesn't work
scott.getHobbiesPublic(); // works

09:07 - Class fields

  • This may seem super old because we have been polyfilling it forever
  • Right now if you want an instance field on a class, you need to declare it in the constructor
  • Now we can just declare them inside the class

10:36 - Static fields and methods

  • As above can also be static with the static keyboard
  • Works for methods too
    • Explain what a static method is

13:17 - Top level await

  • So handy in modules. Need to pull in some data? Simple.

15:19 - Ergonomic brand checks for private fields

  • Used for checking if a private field on a class exists using the in keyword

16:00 - .at() method

  • Strings and arrays - we can use square brackets to reference items of the array
  • Super handy for grabbing the last item of an array
// 🔥 New .at() method on arrays and strings

const toppings = ['pepperoni', 'cheese', 'mushrooms'];

// The old way to grab the last item
toppings[toppings.length - 1]; // mushrooms

// using .at() method with a negative index
toppings.at(-1); // mushrooms 
// works with any index
toppings.at(0); // pepperoni
toppings.at(-2); // cheese

// and with strings!
'Meeting Room: B'.at(-1) // B
  • Why not use array[-1]?
  • We used to use slice(-1)
  • What about indexOf?

21:34 - Handy hasOwn method

24:51 - Class static block


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