循环语句 #

for (初始化; 条件; 更新) {
  // 循环体
}

for (let i = 0; i < 5; i++) {
  console.log(i);
}
// 输出：0 1 2 3 4

for (let i = 5; i > 0; i--) {
  console.log(i);
}
// 输出：5 4 3 2 1

for (let i = 0; i < 10; i += 2) {
  console.log(i);
}
// 输出：0 2 4 6 8

for (let i = 0, j = 10; i < j; i++, j--) {
  console.log(`i: ${i}, j: ${j}`);
}
// 输出：i: 0, j: 10 ... i: 4, j: 6

let i = 0;
for (; i < 5; ) {
  console.log(i);
  i++;
}

while (条件) {
  // 循环体
}

let count = 0;

while (count < 5) {
  console.log(count);
  count++;
}
// 输出：0 1 2 3 4

const inputs = ["hello", "world", "quit"];
let index = 0;

while (inputs[index] !== "quit") {
  console.log(inputs[index]);
  index++;
}
// 输出：hello world

while (true) {
  const random = Math.random();
  console.log(random);
  
  if (random > 0.9) {
    break;
  }
}

do {
  // 循环体
} while (条件);

let count = 0;

do {
  console.log(count);
  count++;
} while (count < 5);
// 输出：0 1 2 3 4

let value = 10;

do {
  console.log(value);
  value++;
} while (value < 5);
// 输出：10（至少执行一次）

function getValidInput(): string {
  let input: string;
  
  do {
    input = "valid"; // 模拟输入
  } while (input.length < 3);
  
  return input;
}

for (const item of iterable) {
  // 循环体
}

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

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

const str = "Hello";

for (const char of str) {
  console.log(char);
}
// 输出：H e l l o

const map = new Map([
  ["a", 1],
  ["b", 2],
  ["c", 3]
]);

for (const [key, value] of map) {
  console.log(`${key}: ${value}`);
}
// 输出：a: 1 b: 2 c: 3

const set = new Set([1, 2, 3, 2, 1]);

for (const value of set) {
  console.log(value);
}
// 输出：1 2 3

function* range(start: number, end: number) {
  for (let i = start; i <= end; i++) {
    yield i;
  }
}

for (const num of range(1, 5)) {
  console.log(num);
}
// 输出：1 2 3 4 5

for (const key in object) {
  // 循环体
}

const user = {
  name: "Alice",
  age: 25,
  city: "Beijing"
};

for (const key in user) {
  console.log(`${key}: ${user[key as keyof typeof user]}`);
}
// 输出：name: Alice age: 25 city: Beijing

const arr = ["a", "b", "c"];

for (const index in arr) {
  console.log(`${index}: ${arr[index as number]}`);
}
// 输出：0: a 1: b 2: c

const arr = ["a", "b", "c"];

// for...in：遍历索引（键）
for (const key in arr) {
  console.log(key); // 0, 1, 2
}

// for...of：遍历值
for (const value of arr) {
  console.log(value); // a, b, c
}

const numbers = [1, 2, 3, 4, 5];

numbers.forEach((num, index) => {
  console.log(`索引 ${index}: ${num}`);
});

const numbers = [1, 2, 3, 4, 5];

const doubled = numbers.map(num => num * 2);
console.log(doubled); // [2, 4, 6, 8, 10]

const numbers = [1, 2, 3, 4, 5];

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

const numbers = [1, 2, 3, 4, 5];

const sum = numbers.reduce((acc, num) => acc + num, 0);
console.log(sum); // 15

const users = [
  { id: 1, name: "Alice" },
  { id: 2, name: "Bob" },
  { id: 3, name: "Charlie" }
];

const user = users.find(u => u.id === 2);
console.log(user); // { id: 2, name: "Bob" }

const index = users.findIndex(u => u.name === "Charlie");
console.log(index); // 2

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

for (let i = 0; i < matrix.length; i++) {
  for (let j = 0; j < matrix[i].length; j++) {
    console.log(`matrix[${i}][${j}] = ${matrix[i][j]}`);
  }
}

for (let i = 1; i <= 9; i++) {
  let line = "";
  for (let j = 1; j <= i; j++) {
    line += `${j}×${i}=${i * j}\t`;
  }
  console.log(line);
}

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

function findInMatrix(matrix: number[][], target: number): [number, number] | null {
  for (let i = 0; i < matrix.length; i++) {
    for (let j = 0; j < matrix[i].length; j++) {
      if (matrix[i][j] === target) {
        return [i, j];
      }
    }
  }
  return null;
}

console.log(findInMatrix(matrix, 5)); // [1, 1]

const items = Array.from({ length: 100 }, (_, i) => `Item ${i + 1}`);
const pageSize = 10;

for (let page = 0; page < Math.ceil(items.length / pageSize); page++) {
  const start = page * pageSize;
  const end = start + pageSize;
  const pageItems = items.slice(start, end);
  
  console.log(`Page ${page + 1}:`, pageItems);
}

async function processBatch<T>(
  items: T[],
  batchSize: number,
  processor: (item: T) => Promise<void>
): Promise<void> {
  for (let i = 0; i < items.length; i += batchSize) {
    const batch = items.slice(i, i + batchSize);
    await Promise.all(batch.map(processor));
    console.log(`Processed ${Math.min(i + batchSize, items.length)}/${items.length}`);
  }
}

async function fetchWithRetry(
  url: string,
  maxRetries: number = 3
): Promise<Response> {
  let lastError: Error | null = null;
  
  for (let attempt = 1; attempt <= maxRetries; attempt++) {
    try {
      const response = await fetch(url);
      if (response.ok) {
        return response;
      }
      throw new Error(`HTTP ${response.status}`);
    } catch (error) {
      lastError = error as Error;
      console.log(`Attempt ${attempt} failed: ${lastError.message}`);
      
      if (attempt < maxRetries) {
        await new Promise(resolve => setTimeout(resolve, 1000 * attempt));
      }
    }
  }
  
  throw lastError;
}

async function pollUntil(
  checkFn: () => Promise<boolean>,
  interval: number = 1000,
  timeout: number = 30000
): Promise<boolean> {
  const startTime = Date.now();
  
  while (Date.now() - startTime < timeout) {
    if (await checkFn()) {
      return true;
    }
    await new Promise(resolve => setTimeout(resolve, interval));
  }
  
  return false;
}

const arr = [1, 2, 3, 4, 5];

// 不推荐：每次循环都计算length
for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

// 推荐：缓存length
for (let i = 0, len = arr.length; i < len; i++) {
  console.log(arr[i]);
}

// 遍历数组：使用for...of
for (const item of array) { }

// 遍历对象：使用for...in或Object方法
for (const key in object) { }
Object.keys(object).forEach(key => { });
Object.entries(object).forEach(([key, value]) => { });

// 需要索引：使用for或forEach
for (let i = 0; i < array.length; i++) { }
array.forEach((item, index) => { });