javascript - 生成器函数未产生正确的值

Question

我最近学习了一些 ES6，并使用在 io.js 上运行的 koa.js 开始了我的新项目。

在下面的代码中，我试图检查是否已经有另一个具有相同 url slug 的项目。

但是 counter.next().value 的值始终返回一个函数，因此函数 _slugExists 始终返回 true

'use strict';

let _ = require('lodash');
let config = require('../../config');

let monk = require('monk');
let wrap = require('co-monk');
let db = monk(config.db);

let _apps = wrap(db.get('apps'));

function _validateApp(app) {
  let slug = app.slug.trim();

  if (!app.name.trim()) {
    throw new Error('App name was not specified.');
  }

  if (!slug) {
    throw new Error('App URL was not specified.');
  }

  if (_slugExists(slug)) {
    throw new Error('Another app with the same URL already exists.');
  }
}

function* _count(filter) {
  yield _apps.count(filter);
}

function _slugExists(slug) {
  let counter = _count({
    slug: slug
  });

  return counter.next().value !== 0;
}

module.exports = {
  list: function*(next) {
    this.status = 200;
    this.body = yield _apps.find({});
  },
  create: function*(next) {
    try {
      let app = this.request.body;
      _validateApp(app);

      this.status = 201;
      this.body = {
        id: yield _apps.insert({
          name: app.name.trim(),
          slug: app.slug.trim(),
          created_at: new Date()
        })
      };
    } catch (error) {
      console.log(`[ERROR] ${error.message}`);

      this.status = 500;
      this.body = {
        error: error.message
      };
    }
  }
}

Answer 1

在基于co的koa中，任何异步操作都必须yield promise 一直到koa。您还可以生成生成器，但不能生成迭代器。特别重要的是确保嵌套的异步操作不会挂起:

function* middleware(next) {
  yield Promise.resolve(0); // Yielding a promise. Good.
  yield (function() { return Promise.resolve(0); })(); // Also yielding a promise. Good.

  yield gen(4); // Yielding iterator. NOT GOOD!
  yield gen; // Yielding generator. Good, but no arg.
  yield* gen(4); // Delegating iterator. Good!

  hangingNested(); // Not yielding anything, async is lost. NOT GOOD!
  yield properNested; // Yielding generator with nested delegate, good!
}

function* gen(arg) {
  yield Promise.resolve(1);
  yield Promise.resolve(2);
  yield Promise.resolve(3);
  return arg;
}

function hangingNested() { // not a generator, nothing is yielded outside.
  gen(4); // iterator is lost.
}

function* properNested() {
  yield* gen(4); // Delegating iterator.
}

考虑到这一点，您可以通过多种方式修复代码，例如:

function* _validateApp(app) {
  let slug = app.slug.trim();

  if (!app.name.trim()) {
    throw new Error('App name was not specified.');
  }

  if (!slug) {
    throw new Error('App URL was not specified.');
  }

  if (yield* _slugExists(slug)) {
    throw new Error('Another app with the same URL already exists.');
  }
}

function* _count(filter) {
  return yield _apps.count(filter);
}

function* _slugExists(slug) {
  let counter = yield* _count({
    slug: slug
  });

  return counter !== 0;
}

module.exports = {
  list: function*(next) {
    this.status = 200;
    this.body = yield _apps.find({});
  },
  create: function*(next) {
    try {
      let app = this.request.body;
      yield* _validateApp(app);

      this.status = 201;
      this.body = {
        id: yield _apps.insert({
          name: app.name.trim(),
          slug: app.slug.trim(),
          created_at: new Date()
        })
      };
    } catch (error) {
      console.log(`[ERROR] ${error.message}`);

      this.status = 500;
      this.body = {
        error: error.message
      };
    }
  }
}