深入了解JavaScript中的函数式编程

// 使用map函数将数组中的每个元素加倍
const numbers = [1, 2, 3, 4, 5];
const doubledNumbers = numbers.map(n => n * 2);
console.log(doubledNumbers); // [2, 4, 6, 8, 10]

// 使用compose函数组合多个函数
const compose = (f, g) => x => f(g(x));
const addOne = x => x + 1;
const multiplyByTwo = x => x * 2;
const addOneAndMultiplyByTwo = compose(multiplyByTwo, addOne);
console.log(addOneAndMultiplyByTwo(3)); // 8

const name = 'Alice';
name = 'Bob'; // 报错：Assignment to constant variable

const numbers = [1, 2, 3];
const newNumbers = [...numbers, 4]; // 创建新的数组副本
console.log(newNumbers); // [1, 2, 3, 4]

const person = { name: 'Bob', age: 30 };
const newPerson = Object.assign({}, person, { age: 31 }); // 创建新的对象副本
console.log(newPerson); // { name: 'Bob', age: 31 }

const person = { name: 'Bob', age: 30 };
const newPerson = { ...person, age: 31 }; // 创建新的对象副本
console.log(newPerson); // { name: 'Bob', age: 31 }

// 用户列表
const users = [  { name: 'Alice', age: 25 },  { name: 'Bob', age: 30 },  { name: 'Charlie', age: 35 }];
// 使用命令式编程方式
function sumOfAdultNameLengths(users) {
  let sum = 0;
  for (let i = 0; i < users.length; i++) {
    if (users[i].age >= 18) {
      sum += users[i].name.length;
    }
  }
  return sum;
}
console.log(sumOfAdultNameLengths(users)); // 16
// 使用函数式编程方式
const pipe = (...fns) => x => fns.reduce((v, fn) => fn(v), x);
const filter = predicate => array => array.filter(predicate);
const map = transform => array => array.map(transform);
const prop = key => obj => obj[key];
const reduce = (reducer, initialValue) => array => array.reduce(reducer, initialValue);
const sumOfAdultNameLengths = pipe(
  filter(user => user.age >= 18),
  map(prop('name')),
  map(name => name.length),
  reduce((sum, length) => sum + length, 0)
);
console.log(sumOfAdultNameLengths(users)); // 16