JavaScript更新到了es几

// ✅ a regex with a 'B' named group capture
const expr = /a+(?<B>b+)+c/d;


const result = expr.exec("aaabbbc")


// ✅ shows start-end matches + named group match
console.log(result.indices);
// prints [Array(2), Array(2), groups: {…}]


// ✅ showing the named 'B' group match
console.log(result.indices.groups['B'])
// prints [3, 6]

// users.js
export const users = await fetch('/users/lists');


// usage.js
import { users } from "./users.js";
// ✅ the module will wait for users to be fullfilled prior to executing any code
console.log(users);

const array = [1,2,3,4,5,6]


// ✅ When used with positive index it is equal to [index]
array.at(0) // 1
array[0] // 1


// ✅ When used with negative index it mimicks the Python behaviour
array.at(-1) // 6
array.at(-2) // 5
array.at(-4) // 3

const array = [1,2,3,4,5,6];


// ✅ Different ways of accessing the third position
const {3: third} = array; // third = 4
array.at(3) // 4
array[3] // 4

const x = { foo: "bar" };


// ✅ grabbing the hasOwnProperty function from prototype
const hasOwnProperty = Object.prototype.hasOwnProperty


// ✅ executing it with the x context
if (hasOwnProperty.call(x, "foo")) {
  ...
}

const x = { foo: "bar" };


// ✅ using the new Object method
if (Object.hasOwn(x, "foo")) {
  ...
}

async function fetchUserPreferences() {
  try { 
    const users = await fetch('//user/preferences')
      .catch(err => {
        // What is the best way to wrap the error?
        // 1. throw new Error('Failed to fetch preferences ' + err.message);
        // 2. const wrapErr = new Error('Failed to fetch preferences');
        //    wrapErr.cause = err;
        //    throw wrapErr;
        // 3. class CustomError extends Error {
        //      constructor(msg, cause) {
        //        super(msg);
        //        this.cause = cause;
        //      }
        //    }
        //    throw new CustomError('Failed to fetch preferences', err);
      })
    }
}


fetchUserPreferences();

async function fetcUserPreferences() {
  try { 
    const users = await fetch('//user/preferences')
      .catch(err => {
        throw new Error('Failed to fetch user preferences, {cause: err});
      })
    }
}


fetcUserPreferences();

class Foo {
  #iteration = 0;


  increment() {
    this.#iteration++;
  }


  logIteration() {
    console.log(this.#iteration);
  }
}


const x = new Foo();


// ❌ Uncaught SyntaxError: Private field '#iteration' must be declared in an enclosing class
x.#iteration


// ✅ works
x.increment();


// ✅ works
x.logIteration();

class Foo {
  #iteration = 0;


  #auditIncrement() {
    console.log('auditing');
  }


  increment() {
    this.#iteration++;
    this.#auditIncrement();
  }
}


const x = new Foo();


// ❌ Uncaught SyntaxError: Private field '#auditIncrement' must be declared in an enclosing class
x.#auditIncrement


// ✅ works
x.increment();

// ✅ will output 'one two three'
class A {
  static {
      console.log('one');
  }
  static {
      console.log('two');
  }
  static {
      console.log('three');
  }
}

let getPrivateField;


class A {
  #privateField;
  constructor(x) {
    this.#privateField = x;
  }
  static {
    // ✅ it can access any private field
    getPrivateField = (a) => a.#privateField;
  }
}


const a = new A('foo');
// ✅ Works, foo is printed
console.log(getPrivateField(a));

class Foo {
  #brand;


  static isFoo(obj) {
    return #brand in obj;
  }
}


const x = new Foo();


// ✅ works, it returns true
Foo.isFoo(x);


// ✅ works, it returns false
Foo.isFoo({})


// ❌ Uncaught SyntaxError: Private field '#brand' must be declared in an enclosing class
#brand in x