高阶函数

1. 高阶函数概述

1.1 定义

高阶函数（Higher-Order Function）是满足以下条件之一的函数：

• 接受一个或多个函数作为参数
• 返回一个函数作为结果

// 函数作为参数
function applyOperation(arr, operation) {
  return arr.map(operation);
}

const doubled = applyOperation([1, 2, 3], (x) => x * 2);
console.log(doubled); // [2, 4, 6]

// 函数作为返回值
function createMultiplier(factor) {
  return function (number) {
    return number * factor;
  };
}

const triple = createMultiplier(3);
console.log(triple(5)); // 15

1.2 为什么需要高阶函数

2. 内置高阶函数

2.1 Array.prototype.map

const users = [
  { name: 'Alice', age: 25 },
  { name: 'Bob', age: 30 },
  { name: 'Charlie', age: 35 },
];

const names = users.map((user) => user.name);
console.log(names); // ['Alice', 'Bob', 'Charlie']

2.2 Array.prototype.filter

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

const evens = numbers.filter((n) => n % 2 === 0);
console.log(evens); // [2, 4, 6, 8, 10]

// 链式调用
const result = numbers.filter((n) => n % 2 === 0).map((n) => n ** 2);
console.log(result); // [4, 16, 36, 64, 100]

2.3 Array.prototype.reduce

// 求和
const sum = [1, 2, 3, 4, 5].reduce((acc, cur) => acc + cur, 0);
console.log(sum); // 15

// 统计词频
const words = ['apple', 'banana', 'apple', 'cherry', 'banana', 'apple'];
const wordCount = words.reduce((acc, word) => {
  acc[word] = (acc[word] || 0) + 1;
  return acc;
}, {});

// 管道函数
const pipe =
  (...fns) =>
  (x) =>
    fns.reduce((v, f) => f(v), x);
const add1 = (x) => x + 1;
const double = (x) => x * 2;
const transform = pipe(add1, double);
console.log(transform(5)); // 12

2.4 其他内置高阶函数

3. 自定义高阶函数

3.1 函数组合

const compose =
  (...fns) =>
  (x) =>
    fns.reduceRight((v, f) => f(v), x);
const pipe =
  (...fns) =>
  (x) =>
    fns.reduce((v, f) => f(v), x);

const add10 = (x) => x + 10;
const multiply3 = (x) => x * 3;
const toString = (x) => `Result: ${x}`;

const compute = pipe(add10, multiply3, toString);
console.log(compute(5)); // "Result: 45"

3.2 记忆化

function memoize(fn) {
  const cache = new Map();
  return function (...args) {
    const key = JSON.stringify(args);
    if (cache.has(key)) return cache.get(key);
    const result = fn.apply(this, args);
    cache.set(key, result);
    return result;
  };
}

const fibonacci = memoize(function (n) {
  if (n <= 1) return n;
  return fibonacci(n - 1) + fibonacci(n - 2);
});

console.log(fibonacci(50)); // 12586269025

3.3 节流与防抖

function debounce(fn, delay) {
  let timer = null;
  return function (...args) {
    clearTimeout(timer);
    timer = setTimeout(() => fn.apply(this, args), delay);
  };
}

function throttle(fn, interval) {
  let lastTime = 0;
  return function (...args) {
    const now = Date.now();
    if (now - lastTime >= interval) {
      lastTime = now;
      fn.apply(this, args);
    }
  };
}

4. 性能考量