从 Promise 结果中导出 Node 模块

问题描述

我正在尝试重写一个模块以返回与以前不同的值，但现在它使用异步调用来获取该值.(如果重要，请使用 child_process).我已经将它包装在一个 Promise 中，但这对我来说并不重要——它可以在原始的 child_process 回调中，但问题是我无法将 Promise 链接到应用程序中的任何地方，因为我需要它变得同步.这是我的模块:

I'm trying to rewrite a module to return a different value than before, but now it uses an async call to get that value. (with child_process if it matters). I've wrapped it in a Promise, but that's not critical to me - it can be in the original child_process callback, but the problem is I can't chain the promise to everywhere in the app because I need this to become synchronous. Here's my module:

exec = require('child_process').exec

platformHome = process.env[if process.platform is 'win32' then 'USERPROFILE' else 'HOME']

getExecPath = new Promise (resolve, reject) ->
  path = process.env.GEM_HOME

  if path
    resolve(path)
  else
    exec 'gem environment', (err, stdout, stderr) ->
      unless err
        line = stdout.split(/
?
/)
                 .find((l) -> ~l.indexOf('EXECUTABLE DIRECTORY'))
        if line
          resolve line[line.indexOf(': ') + 2..]
        else
          reject undefined

GEM_HOME = undefined

getExecPath.then (path) ->
  GEM_HOME = path
.catch ->
  GEM_HOME = "#{platformHome}/.gem/ruby/2.3.0"
.then =>
  module.exports = GEM_HOME // or simply return it

显然，当需要模块时，这不起作用 - 如果我返回承诺本身，并在 require 之后使用 then - 我的下一个 module.exports 将是异步的，并且该链将继续进行.我该如何避免这种模式?

Clearly, when requiring the module, this doesn't work - and if I return the promise itself, and use then after require - my next module.exports will be async, and this chain will carry on. How do I avoid this pattern?

推荐答案

Node 中使用 require() 加载的模块是同步加载的，require 返回任何异步加载的值.它可以返回一个承诺，但该模块的用户必须将其用作:

Modules in Node that you load with require() are loaded synchronously and it is not possible for require to return any value that is loaded asynchronously. It can return a promise but then users of that module would have to use it as:

require('module-name').then(value => {
  // you have your value here
});

写不出来:

var value = require('module-name');
// you cannot have your value here because this line
// will get evaluated before that value is available

当然，你可以在你的模块中解析承诺，并通过添加如下内容使其在导出的对象上设置一个属性:

Of course you can have the promise resolved inside of your module and make it set a property on the exported object by adding something like this:

module.exports = { GEM_HOME: null };

和变化:

module.exports = GEM_HOME

到:

module.exports.GEM_HOME = GEM_HOME

在这种情况下，使用此模块的所有其他模块:

In that case, every other module that uses this module as:

var x = require('module-name');

将 x.GEM_HOME 最初设置为 null 但它最终会在一段时间后更改为正确的值.但它不会立即可用，因为 require() 在 promise 确定并设置值之前返回.

will have x.GEM_HOME originally set to null but it would eventually get changed to a correct value some time later. It would not be available right away though, because require() returns before the promise is settled and the value is set.

目前正在进行讨论，以引入可能适合您的用例的不同语法和语义的异步模块加载.这是一个有争议的主题，值得阅读其背后的所有基本原理 - 请参阅:

There is an ongoing discussion to introduce asynchronous module loading with different syntax and semantics that may be suited for your use case. It's a controversial subjects and it's worth reading all of the rationale behind it - see:

• Node.js、TC-39 和模块 来自 IBM 的 James M Snell
• ES6 模块互操作性 - 节点.js 增强建议
• 为.js辩护 -Dave Herman、Yehuda Katz 和 Caridy Patio 提出的 Node.js 模块提案
• 关于node-eps的Pull Request #3的讨论(002:ES6 模块互操作)

• Node.js, TC-39, and Modules by James M Snell from iBM
• ES6 Module Interoperability - Node.js Enhancement Proposals
• In Defense of .js - A Proposal for Node.js Modules by Dave Herman, Yehuda Katz and Caridy Patio
• Discussion on the Pull Request #3 of node-eps (002: ES6 module interop)

有关详细信息，另请参阅此答案:

See also this answer for more details:

• javascript - 为什么有同步和异步模块的规范吗?

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