[Javascirpt] What’s new in JavaScript (Google I/O ’19)

Private variable in class:

class Counter {
  #count = 0; // cannot be access publicly
 
  get value () {
    return this.#count;
  }
 
  incremenet(){
    this.#count++;
  }
}

class field syntax:

Previously you need to call 'super'

class Animal {
  constructor (name) {
    this.name = name
  }
}
 
class Cat extends Animal {
  constructor (name) {
    super(name);
    this.likesBaths = false;
  }
  meow () {
    // ...
  }
}

But now, we don't need to, you can use classs field:

class Animal {
  constructor (name) {
    this.name = name
  }
}
 
class Cat extends Animal {
  likesBaths = false;
  meow () {
    // ...
  }
}

Above two features are supported in latest Chrome and Node.js.

flatMap:

const duplicate = x => [x, x];
console.log([2,3,4].flatMap(duplicate)); // [2, 2, 3, 3, 4, 4]

Object.fromEntries:

Reverse opreation of Object.entries, it transform [['x', 42], ['y', 50]] too {x: 42, y: 50}:

const object = {x: 42, y: 50, abc: 9001}
const result = Object.fromEntries(
  Object.entries(object)
    .filter(([key, value]) => key.length === 1) // only take x, y, ingore abc
    .map(([key, value]) => [key, value * 2])
) // {x: 84, y: 100}

It is also support to transform back and from Javascirpt Map.

globalThis:

using 'globalThis' to handle all the envs.

Intl: