如何等待 async.parallel？

Question

假设我在下面有这个async.parallel 代码示例，用于并行运行异步函数并等待它们完成，然后再对结果运行代码。

async.parallel({
  one: async (callback) => {/* ... some async code */ return someResult},
  two: async (callback) => {/* ... some async code */ return someResult},
  three: async (callback) => {/* ... some async code */ return someResult},
},(err,results) => {
  if (err) { /* … error handling code */ }
  /* … continued code */
  debug(results)      // prints "results" object successfully without problems
});

我想做的是将继续代码 （在传递给async.parallel的所有函数完成后继续/运行的代码）在async.parallel之外。

我试过了：

const results = await async.parallel({
  one: async (callback) => {/* ... some async code */ return someResult},
  two: async (callback) => {/* ... some async code */ return someResult},
  three: async (callback) => {/* ... some async code */ return someResult},
},(err,results) => {
  if (err) { /* … error handling code */ }
  debug("test print1: from inside async.parallel callback function");
        // called/printed after "test print2" meaning that async.parallel isn't awaited.
  return results;
});

/* … continued code */
debug("test print2: from outside async.parallel");
    // printed before "test print1" meaning that async.parallel isn't awaited.
debug(results)      // prints "undefined"

还有这个：

let results_outer;
await async.parallel({
  one: async (callback) => {/* ... some async code */ return someResult},
  two: async (callback) => {/* ... some async code */ return someResult},
  three: async (callback) => {/* ... some async code */ return someResult},
},(err,results) => {
  if (err) { /* … error handling code */ }
  debug("test print1: from inside async.parallel callback function");
        // printed after "test print2" meaning that async.parallel isn't awaited.
  results_outer = results;
});

/* … continued code */
debug("test print2: from outside async.parallel");
    // printed before "test print1" meaning that async.parallel isn't awaited.
debug(results_outer)      // prints "undefined"

我的问题是，我怎样才能等待async.parallel 并成功获取结果对象？

---

建议的解决方案/替代方案： （目前还不是我的问题的解决方案，但它们可能是有用的临时替代方案）

备选方案 1： （我不喜欢，因为它看起来比 async.parallel 更丑/麻烦）

const results = await Promise.all([
  new Promise( (resolve, reject) => {/* ... some async code */ resolve('some result 1')} ),
  new Promise( (resolve, reject) => {/* ... some async code */ resolve('some result 2')} ),
  new Promise( (resolve, reject) => {/* ... some async code */ resolve('some result 3')} )
]);
debug(results);      // prints "results" object successfully without problems

关于备选方案 1 的说明：
为了使备选方案 1 更简单，我们可以创建 promisify(some_function):

const results = await Promise.all([
  promisfy(some_functionOne),
  promisfy(some_functionTwo),
  promisfy(some_functionThree)
]);
debug(results);      // prints "results" object successfully without problems

function promisfy(some_function) {
  const promise = new Promise( (resolve, reject) => {
    some_function_return_value = await some_function();
    resolve(some_function_return_value);
  } );

  return promise;
}

进一步简化备选方案 1：

async_parallel([some_functionOne, some_functionTwo, some_functionThree]);
debug(results);      // prints "results" object successfully without problems

function async_parallel(functions_to_run) {
  let promises = functions_to_run.map(function_to_run => promisfy(function_to_run));
  const results = await Promise.all(promises);
  return results;
}

我们最终可能会构建一个类似于 异步库 的库并重新发明轮子，特别是当我们需要 concat、waterfall 等功能时。

Answer 1

由于您的数据库的现代版本已经支持 Promise，因此您不需要 async 库来执行此操作。相反，您可以这样做：

let results = await Promise.all([
     Car_model.findOne(...), 
     Car_model.findOne(...), 
     Car_model.findOne(...)
]);
console.log(results);

当然，此代码需要在 async 函数内，以便您可以使用 await。

Answer 2

只要去掉回调函数，async.parallel 就会返回一个Promise。

如果没有通过回调，async.parallel 返回一个承诺。

Source

const results = await async.parallel({
  one: async (callback) => {/* ... some async code */ return someResult},
  two: async (callback) => {/* ... some async code */ return someResult},
  three: async (callback) => {/* ... some async code */ return someResult},
});
debug(results)      // prints "results" object successfully without problems

要处理错误，请将代码放入 try..catch:

try {
  let results = await async.parallel({
    one: async (callback) => {/* ... some async code */ return someResult},
    two: async (callback) => {/* ... some async code */ return someResult},
    three: async (callback) => {/* ... some async code */ return someResult},
  });
  debug(results);
} catch(err) {
  /* … error handling code */
}