Exploring JavaScript's Top-Level Await and New Features

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

• Getters and setters introduced in es5
• https://www.w3schools.com/js/js_object_accessors.asp

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

• https://github.com/tc39/proposal-accessible-object-hasownproperty

24:51 - Class static block

• A static block allows you to run code before creating an optional static property during initialization
• https://github.com/tc39/proposal-class-static-block

Links

• https://github.com/tc39/proposals/blob/master/finished-proposals.md

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