javascript - 在Javascript中等待一些异步任务完成的最简单方法？

Question

我想删除一些 mongodb 集合，但这是一个异步任务。代码将是:

var mongoose = require('mongoose');

mongoose.connect('mongo://localhost/xxx');

var conn = mongoose.connection;

['aaa','bbb','ccc'].forEach(function(name){
    conn.collection(name).drop(function(err) {
        console.log('dropped');
    });
});
console.log('all dropped');

控制台显示:

all dropped
dropped
dropped
dropped

确保在所有集合被删除后打印 all dropped 的最简单方法是什么？任何 3rd-party 都可以用来简化代码。

Answer 1

使用 Promises .

var mongoose = require('mongoose');

mongoose.connect('your MongoDB connection string');
var conn = mongoose.connection;

var promises = ['aaa', 'bbb', 'ccc'].map(function(name) {
  return new Promise(function(resolve, reject) {
    var collection = conn.collection(name);
    collection.drop(function(err) {
      if (err) { return reject(err); }
      console.log('dropped ' + name);
      resolve();
    });
  });
});

Promise.all(promises)
.then(function() { console.log('all dropped)'); })
.catch(console.error);

这会丢弃每个集合，在每个集合之后打印“dropped”，然后在完成时打印“all dropped”。如果发生错误，则显示到 stderr。

---

上一个答案(这早于 Node 对 Promises 的原生支持):

使用 Q promise 或 Bluebird promise 。

用Q:

var Q = require('q');
var mongoose = require('mongoose');

mongoose.connect('your MongoDB connection string');
var conn = mongoose.connection;

var promises = ['aaa','bbb','ccc'].map(function(name){
    var collection = conn.collection(name);
    return Q.ninvoke(collection, 'drop')
      .then(function() { console.log('dropped ' + name); });
});

Q.all(promises)
.then(function() { console.log('all dropped'); })
.fail(console.error);

与 bluebird :

var Promise = require('bluebird');
var mongoose = Promise.promisifyAll(require('mongoose'));

mongoose.connect('your MongoDB connection string');
var conn = mongoose.connection;

var promises = ['aaa', 'bbb', 'ccc'].map(function(name) {
  return conn.collection(name).dropAsync().then(function() {
    console.log('dropped ' + name);
  });
});

Promise.all(promises)
.then(function() { console.log('all dropped'); })
.error(console.error);