JavaScript 中的迭代器和生成器是什么，它们有什么用途？

const iterator = {
  current: 0,
  last: 5,
  next() {
    if (this.current <= this.last) {
      return { value: this.current++, done: false };
    } else {
      return { value: undefined, done: true };
    }
  },
};

let result = iterator.next();
while (!result.done) {
  console.log(result.value); // Logs 0, 1, 2, 3, 4, 5
  result = iterator.next();
}

function* numberGenerator() {
  let num = 0;
  while (num <= 5) {
    yield num++;
  }
}

const gen = numberGenerator();
console.log(gen.next()); // { value: 0, done: false }
console.log(gen.next()); // { value: 1, done: false }
console.log(gen.next()); // { value: 2, done: false }
console.log(gen.next()); // { value: 3, done: false }
console.log(gen.next()); // { value: 4, done: false }
console.log(gen.next()); // { value: 5, done: false }
console.log(gen.next()); // { value: undefined, done: true }

// 定义一个名为 Range 的类
class Range {
  // 构造函数接受两个参数：start 和 end
  constructor(start, end) {
    // 将 start 和 end 值分配给实例
    this.start = start;
    this.end = end;
  }

  // 定义对象的默认迭代器
  [Symbol.iterator]() {
    // 将当前值初始化为起始值
    let current = this.start;
    const end = this.end;

    // 返回一个带有 next 方法的对象
    return {
      // next 方法返回迭代中的下一个值
      next() {
        // 如果当前值小于或等于结束值...
        if (current <= end) {
          // ...返回一个带有当前值和 done 设置为 false 的对象
          return { value: current++, done: false };
        }

        // ...否则，返回一个 value 设置为 undefined 且 done 设置为 true 的对象
        return { value: undefined, done: true };
      },
    };
  }
}

// 创建一个 start = 1 且 end = 3 的新 Range 对象
const range = new Range(1, 3);
// 迭代 range 对象
for (const number of range) {
  // 将每个数字记录到控制台
  console.log(number); // 1, 2, 3
}

// Define a class named Range
class Range {
  // The constructor takes two parameters: start and end
  constructor(start, end) {
    // Assign the start and end values to the instance
    this.start = start;
    this.end = end;
  }

  // Define the default iterator for the object using a generator
  *[Symbol.iterator]() {
    // Initialize the current value to the start value
    let current = this.start;

    // While the current value is less than or equal to the end value...
    while (current <= this.end) {
      // ...yield the current value
      yield current++;
    }
  }
}

// Create a new Range object with start = 1 and end = 3
const range = new Range(1, 3);
// Iterate over the range object
for (const number of range) {
  // Log each number to the console
  console.log(number); // 1, 2, 3
}

async function* fetchDataInBatches(url, numBatches = 5, batchSize = 10) {
  let startIndex = 0;
  let currBatch = 0;

  while (currBatch < numBatches) {
    const response = await fetch(
      `${url}?_start=${startIndex}&_limit=${batchSize}`,
    );
    const data = await response.json();
    if (data.length === 0) break;
    yield data;
    startIndex += batchSize;
    currBatch += 1;
  }
}

async function fetchAndLogData() {
  const dataGenerator = fetchDataInBatches(
    'https://jsonplaceholder.typicode.com/todos',
  );

  for await (const batch of dataGenerator) {
    console.log(batch);
  }
}
fetchAndLogData();

async function* fetchDataAsyncIterator(url, pagesToFetch = 3) {
  let currPage = 1;

  while (currPage <= pagesToFetch) {
    const response = await fetch(`${url}?_page=${currPage}`);
    const data = await response.json();
    if (data.length === 0) break;
    yield data;
    currPage++;
  }
}

async function fetchAndLogData() {
  const asyncIterator = fetchDataAsyncIterator(
    'https://jsonplaceholder.typicode.com/todos',
  );

  for await (const chunk of asyncIterator) {
    console.log(chunk);
  }
}
fetchAndLogData();