javascript - 涵盖 for 循环和异步回调的通用解决方案

Question

我的代码中有几个场景符合这种模式，我确信这是一个常见的编程问题。我想遍历一个数组或一个对象，并在每次迭代时运行一个(可能)异步函数。在当前迭代完成之前，我不希望下一次迭代开始。

如果处理是同步的，下面是可以工作的代码

    var k;
    for (k in data) {
        var thisdata = data[k];
        dosomething(thisdata);
    }

上面dosomething可能会访问服务器，也可能会抛出一个对话框让用户输入，也可能只是在js中做一些本地处理。

这是我想出的解决方案:

    var keys = Object.keys(data);
    var index = 0;
    exec();

    function exec() {
        if (index == keys.length) return;
        var k = keys[index++];
        var thisdata = data[k];
        dosomething(thisdata, exec);
    }

因此必须修改 dosomething() 以进行回调，这很好，但是使用 Object.keys 并使用如此不雅且难以阅读的代码似乎是错误的。那么有没有更简单/更好的方法？

这个循环也可能有数千次迭代，所以“stackoverflow”是一个问题，我想，即。正在进行 1000 次深度递归。

Answer 1

如果您有可能使用转译器，我建议您查看 ES7 async/await 提案。您可以像这样编码，然后通过 Babel(或 Facebook Regenerator)将其编译:

const doSomething = (param) => {
  return new Promise((resolve, reject) => {
    // in my example, I'll do an AJAX request
    const xhr = new XMLHttpRequest();
    xhr.addEventListener('load', (res) => {
      if (xhr.status >= 200 && xhr.status < 300)
        resolve(JSON.parse(xhr.responseText));
      else
        reject(xhr.statusText);
    });
    xhr.addEventListener('error', (err) => {
      reject(err);
    });
    xhr.open('GET', 'https://api.github.com/repos/buzinas/tslint-eslint-rules/issues/' + param);
    xhr.send();
  });
}

const getData = async (data) => { 
  for (let d of data) {
    let result = await doSomething(d);
    document.querySelector('ul').insertAdjacentHTML('beforeend', `<li>${d}: ${result.title} - ${result.state}</li>`);
  }
};

getData([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);

---

看看下面运行的转译代码:

'use strict';

var _this = this;

var doSomething = function doSomething(param) {
  return new Promise(function (resolve, reject) {
    // in my example, I'll do an AJAX request
    var xhr = new XMLHttpRequest();
    xhr.addEventListener('load', function (res) {
      if (xhr.status >= 200 && xhr.status < 300) resolve(JSON.parse(xhr.responseText));else reject(xhr.statusText);
    });
    xhr.addEventListener('error', function (err) {
      reject(err);
    });
    xhr.open('GET', 'https://api.github.com/repos/buzinas/tslint-eslint-rules/issues/' + param);
    xhr.send();
  });
};

var getData = function getData(data) {
  var _iteratorNormalCompletion, _didIteratorError, _iteratorError, _iterator, _step, d, result;

  return regeneratorRuntime.async(function getData$(context$1$0) {
    while (1) switch (context$1$0.prev = context$1$0.next) {
      case 0:
        _iteratorNormalCompletion = true;
        _didIteratorError = false;
        _iteratorError = undefined;
        context$1$0.prev = 3;
        _iterator = data[Symbol.iterator]();

      case 5:
        if (_iteratorNormalCompletion = (_step = _iterator.next()).done) {
          context$1$0.next = 14;
          break;
        }

        d = _step.value;
        context$1$0.next = 9;
        return regeneratorRuntime.awrap(doSomething(d));

      case 9:
        result = context$1$0.sent;

        document.querySelector('ul').insertAdjacentHTML('beforeend', '<li>' + d + ': ' + result.title + ' - ' + result.state + '</li>');

      case 11:
        _iteratorNormalCompletion = true;
        context$1$0.next = 5;
        break;

      case 14:
        context$1$0.next = 20;
        break;

      case 16:
        context$1$0.prev = 16;
        context$1$0.t0 = context$1$0['catch'](3);
        _didIteratorError = true;
        _iteratorError = context$1$0.t0;

      case 20:
        context$1$0.prev = 20;
        context$1$0.prev = 21;

        if (!_iteratorNormalCompletion && _iterator['return']) {
          _iterator['return']();
        }

      case 23:
        context$1$0.prev = 23;

        if (!_didIteratorError) {
          context$1$0.next = 26;
          break;
        }

        throw _iteratorError;

      case 26:
        return context$1$0.finish(23);

      case 27:
        return context$1$0.finish(20);

      case 28:
      case 'end':
        return context$1$0.stop();
    }
  }, null, _this, [[3, 16, 20, 28], [21,, 23, 27]]);
};

getData([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);

<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.33/browser-polyfill.min.js"></script>

<ul>

</ul>