一次性让大家搞懂JavaScript数组方法reduce、every、some

// 求数组元素乘积
const nums = [2, 3, 4];
const product = nums.reduce((acc, num) => acc * num, 1); // 24

// 数组转对象映射
const users = ['Alice', 'Bob'];
const map = users.reduce((acc, user) => {
  acc[user] = true;
  return acc;
}, {}); // { Alice: true, Bob: true }

function myReduce(arr, callback, initialValue) {
  let acc = initialValue ?? arr[0];
  let start = initialValue === undefined ? 1 : 0;
  
  for (let i = start; i < arr.length; i++) {
    acc = callback(acc, arr[i], i, arr);
  }
  return acc;
}

// 是否全为偶数
const nums = [2, 4, 6, 7];
const allEven = nums.every(num => num % 2 === 0); // false

// 是否全为有效用户
const users = [{age: 18}, {age: 20}];
const allValid = users.every(user => user.age >= 18); // true

function myEvery(arr, callback) {
  for (let i = 0; i < arr.length; i++) {
    if (!callback(arr[i], i, arr)) return false;
  }
  return true;
}

// 是否存在负数
const nums = [3, -1, 5];
const hasNegative = nums.some(num => num < 0); // true

// 是否有管理员用户
const users = [{role: 'user'}, {role: 'admin'}];
const hasAdmin = users.some(user => user.role === 'admin'); // true

function mySome(arr, callback) {
  for (let i = 0; i < arr.length; i++) {
    if (callback(arr[i], i, arr)) return true;
  }
  return false;
}

const arr = [1, 2, 2, 3];
const unique = arr.reduce((acc, num) => 
  acc.includes(num) ? acc : [...acc, num], []
);

const words = ['HELLO', 'WORLD'];
const isAllUpper = words.every(word => 
  word === word.toUpperCase()
); // true

const nums = [4, 6, 8, 11];
const hasPrime = nums.some(num => {
  for (let i = 2; i <= Math.sqrt(num); i++) {
    if (num % i === 0) return false;
  }
  return num > 1;
}); // true（11是素数）