1a.js

class Counter {
  constructor() {
    this.name = 'Counter';
    this.count = 0;
  }
  
  inc() {
    this.count++;
  }
}

1b.js

class Counter {
  name = 'Counter';
  #count = 0;  // private field!

  inc() {
    this.#count++;
  }
}

2a.js

class Counter {
  name = 'Counter';
  #count = 0;

  static isCounter(obj) {
    return #count in obj;
  }
}

const counter = new Counter();
Counter.isCounter(counter);   // true

3a.js

const A = [2, 4, 6, 8, 10]
A.at(-1)    // 10

const S = "Hello World"
S.at(-1)    // 'd'

4a.js

const A = [1, 20, 3, 40, 5];

function findBackward(A, predicate) {
  for (let i = A.length-1; i>=0; i--) {
    if (predicate(A[i])) {
      return A[i];
    }
  }
  return -1;
}

findBackward(A, x => x % 10 === 0);    // 40

// be careful with this function!
A.reverse().find(x => x % 10 === 0);   // 40

4b.js

const A = [1, 20, 3, 40, 5];

A.find(v => v%10 === 0)     // 20
A.findLast(v => v%10 === 0) // 40

A.findIndex(v => v%10 === 0)     // 1
A.findLastIndex(v => v%10 === 0) // 3
A.findLastIndex(v => v === 0)    // -1

5a.js

let hasOwnProperty = Object.prototype.hasOwnProperty;

if (hasOwnProperty.call(object, 'foo')) {
  console.log('has property foo');
}

5b.js

object.hasOwnProperty('foo')

5c.js

if (Object.hasOwn(object, 'foo')) {
  console.log('has property foo');
}

6a.js

await fetch('https://example.com/data.csv')
  .catch((err) => {
     throw new Error('failed to get: ' + err.message);
  })

6b.js

await fetch('https://example.com/data.csv')
  .catch((err) => {
     throw new Error('failed to get', { cause: err })
  })
  .catch((err) => {
     console.log('cause', err.cause)
  })

7a.js

#!/usr/bin/env node

'use strict';
console.log(1);

7b.js

#!/usr/bin/env node

export {};
console.log(1);