javascript - 从嵌套在函数中的 Node.js 回调返回对象

Question

这个问题已经有答案了:

How do I return the response from an asynchronous call? (42 个回答)

已关闭 8 年前。

假设我有以下函数，其中包含同步文件系统调用:

var foo = function(){

var numbytes = fs.readSync(...);

return numbytes;
}

没有问题。

但是如果我使用带有回调的异步函数，如下所示:

var baz = function(){

var numbytes = fs.read(...,function(){

//   return numbytes; (wrong)
});

 //return numbytes; (wrong)
};

如何从 baz 函数正确返回与 numbytes 相关的值？

在 Meteor Node.js 框架中，这可能是解决此问题的一个答案:

Meteor.wrapAsync(func, [context]) Anywhere

Wrap a function that takes a callback function as its final parameter. On the server, the wrapped function can be used either synchronously (without passing a callback) or asynchronously (when a callback is passed). On the client, a callback is always required; errors will be logged if there is no callback. If a callback is provided, the environment captured when the original function was called will be restored in the callback.
Arguments

func Function

    A function that takes a callback as its final parameter
context Object

    Optional this object against which the original function will be invoked

Answer 1

处理这个问题的正常方法是将外部函数也变成异步函数。如果您无法控制调用函数的生命周期，但可以控制稍后使用其返回值的位置，则可以使用 Promise 来表示内部异步函数的 future 值，并“解开”该 Promise当需要时。

例如，使用Bluebird promise 库:

var fs = require("fs");
var Promise = require("bluebird");

var baz = function(){

    var promise = new Promise(function(resolve, reject){
        fs.readFile("file.json", function(err, val) {
            if (err) {
                reject(err);
            }
            else {
                resolve(val);
            }
        });
    });
    return promise;
});


var bazPromise = baz();
bazPromise.then(function(value){
    // deal with value
}, function(error){
    // deal with error
});