The ES2020 features

Private variable in Class

class MyClass {
    #privateVariable = "Hello private world";

    helloWorld() {
        console.info(this.#privateVariable);
    }
}

const myClass = new MyClass();
myClass.helloWorld(); // works
console.info(myClass.#helloWorld); // SyntaxError: Private field '#helloWorld' must be declared in an enclosing class

BigInt

const maxInteger = Number.MAX_SAFE_INTEGER;

console.info(maxInteger); // 9007199254740991
console.info(maxInteger + 1); // 9007199254740992
console.info(maxInteger + 2); // 9007199254740992 ??
console.info(maxInteger + 3); // 9007199254740994
console.info(maxInteger + 200); // 9007199254741192 ??
console.info(maxInteger * 200); // 1801439850948198100 ??

Promise.allSettled()

const resolvingPromise1000ms = new Promise((resolve, reject) =>
    setTimeout(resolve, 1000)
);
const rejectingPromise2000ms = new Promise((resolve, reject) =>
    setTimeout(reject, 2000)
);

const timeCheckpoint = Date.now();
Promise.allSettled([resolvingPromise1000ms, rejectingPromise2000ms]).then(
    (data) => {
        const elapsedTimeInMS = Date.now() - timeCheckpoint;
        console.info(`Promise.allSettled resolved after ${elapsedTimeInMS}ms`);
        console.info(data);
    }
);

/*
Promise.allSettled resolved after 2006ms // ? not sure why we have 6ms
[
  { status: 'fulfilled', value: undefined },
  { status: 'rejected', reason: undefined }
]
*/

Nullish Coalescing Operator

let object = {
    car: {
        speed: 0,
        name: "",
    },
};

console.info(object.car.speed || 90); // 90
console.info(object.car.speed ?? 90); // 0

console.info(null || true); // true
console.info(null ?? true); // true

console.info(undefined || true); // true
console.info(undefined ?? true); // true

console.info(0 || true); // true
console.info(0 ?? true); // 0

console.info("" || true); // true
console.info("" ?? true); // ""

console.info([] || true); // []
console.info([] ?? true); // []

console.info({} || true); // {}
console.info({} ?? true); // {}

console.info(true || "hey"); // true
console.info(true ?? "hey"); // true

console.info(false || true); // true
console.info(false ?? true); // false

Optional Chaining Operator

let person = {
    name: "John",
    age: 20,
};

if (person.city !== undefined && person.city.locale !== undefined) {
    const cityLocale = person.city.locale;
}

console.info(person?.city?.locale);

Dynamic Import

print.js

const print = (value) => console.info(value);

export { print };

const doPrint = async (value) => {
    const Print = await import("./print.js");
    Print.print(value);
};

doPrint("Dynamic import works !");

String.prototype.matchAll

const re = /(Mister )\w+/g;
const str = "Mister Smith with Mister Galladon";
const matches = str.matchAll(re);

console.info(matches); // Object [RegExp String Iterator] {}
console.info(Array.from(matches));
/*
[
  [
    'Mister Smith',
    'Mister ',
    index: 0,
    input: 'Mister Smith with Mister Galladon',
    groups: undefined
  ],
  [
    'Mister Galladon',
    'Mister ',
    index: 18,
    input: 'Mister Smith with Mister Galladon',
    groups: undefined
  ]
]
*/