从承诺结果导出节点模块 [英] Exporting Node module from promise result

问题描述

我正在尝试重写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。 （如果重要的话，用 child_process ）。我把它包装在Promise中，但这对我来说并不重要 - 它可以在原始的child_process回调中，但问题是我无法将承诺链接到应用程序中的任何地方，因为我需要这个变为同步。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(/\r?\n/)
                 .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 - 如果我自己返回承诺，并在 require 之后使用然后 - 我的下一个 module.exports 将是异步的，这个链将继续。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

加载 require（）加载的Node中的模块已加载同步并且 require 不可能返回异步加载的任何值。它可以返回一个承诺，但该模块的用户必须使用它：

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
});

不可能写：

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

当然，您可以在模块内部解析承诺，并通过添加以下内容使其在导出的对象上设置属性：

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 };

并更改：

module.exports = GEM_HOME

to：

module.exports.GEM_HOME = GEM_HOME

在这种情况下，使用此模块的每个其他模块：

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

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

最初设置为 x.GEM_HOME null 但最终会在一段时间后更改为正确的值。它不会立即可用，因为 require（）在承诺结算并设置值之前返回。

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.

目前正在讨论引入异步模块加载，其中包含可能适合您的用例的不同语法和语义。这是一个有争议的主题，值得阅读它背后的所有理由 - 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


• ES6模块互操作性 - Node.js增强建议


• 为.js辩护 - 由Dave Herman，Yehuda Katz和CaridyPatiño提出的关于Node.js模块的提案


• 关于提取节点eps的请求＃3 （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 Patiño
• Discussion on the Pull Request #3 of node-eps (002: ES6 module interop)

有关详细信息，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？

• javascript - Why is there a spec for sync and async modules?

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