电子书

// let example
let age = 25;
age = 30; // allowed

// const example
const name = 'john';
name = 'doe'; // error: assignment to constant variable

// regular function
function add(a, b) {
  return a + b;
}

// arrow function
const add = (a, b) => a + b; // implicit return

const name = 'alice';
const greeting = `hello, ${name}!`; // string interpolation

console.log(greeting); // hello, alice!

const numbers = [1, 2, 3];
const [first, second] = numbers;

console.log(first); // 1
console.log(second); // 2

const person = { name: 'john', age: 25 };
const { name, age } = person;

console.log(name); // john
console.log(age); // 25

const greet = (name = 'guest') => `hello, ${name}!`;

console.log(greet()); // hello, guest!
console.log(greet('alice')); // hello, alice!

const sum = (...numbers) => numbers.reduce((acc, num) => acc + num, 0);

console.log(sum(1, 2, 3, 4)); // 10

const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];
const combined = [...arr1, ...arr2];

console.log(combined); // [1, 2, 3, 4, 5, 6]

class person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  greet() {
    console.log(`hello, my name is ${this.name}.`);
  }
}

const john = new person('john', 25);
john.greet(); // hello, my name is john.

// file: math.js
export const add = (a, b) => a + b;
export const subtract = (a, b) => a - b;

// file: app.js
import { add, subtract } from './math';

console.log(add(2, 3)); // 5

const fetchdata = () => {
  return new promise((resolve, reject) => {
    settimeout(() => {
      resolve('data received');
    }, 2000);
  });
};

fetchdata().then(data => console.log(data)); // data received (after 2 seconds)

const fruits = ['apple', 'banana', 'cherry'];

for (const fruit of fruits) {
  console.log(fruit);
}
// apple
// banana
// cherry

const map = new map();
map.set('name', 'john');
map.set('age', 25);

console.log(map.get('name')); // john

const set = new Set([1, 2, 3, 3]); // duplicates are ignored
set.add(4);

console.log(set.has(3)); // true
console.log(set.size); // 4