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The story between node.js and macOS

小云云Original: 2017-12-18 11:22:141472browse

This article uses a short story to share with you the story between node.js and macOS. I hope it can help everyone.

George G did a small test on his computer, but the results were quite different than expected.

So let’s first take a look at what is written in this small test:

There are three files in total, and the total code does not exceed 15 lines

parent.js

<span style="font-size: 14px;">class Parent {}<br><br>module.exports = Parent<br></span>

son.js

<span style="font-size: 14px;">//加载时把模块文件名首字母大写了(不正确的)<br/>const Parent = require(&#39;./Parent&#39;)<br/><br/>class Son extends Parent {}<br/><br/>module.exports = Son<br/></span>

test.js

<span style="font-size: 14px;">//加载时把模块名首字母大写(不正确的)<br/>const ParentIncorrect = require(&#39;./Parent&#39;)<br/>//通过正确的模块文件名加载(正确)<br/>const Parent = require(&#39;./parent&#39;)<br/><br/>const Son = require(&#39;./son&#39;)<br/><br/>const ss = new Son()<br/><br/>//测试结果<br/>console.log(ss instanceof Parent) // false<br/>console.log(ss instanceof ParentIncorrect) // true<br/></span>

Classmate George G has the following questions:

##son.js# Both ## and test.js have incorrect file name references (case issues). Why is no error reported?
Test result, why
ss instanceof ParentIncorrect === true? I tolerated not reporting an error, but why did I still think that I was the instance of the module that was loaded with an incorrect name?

#If you, classmate, already have a clear understanding of the above issues, congratulations, Wen Neng can write a pen to calm the sky, and Wu Neng can mount a horse to determine the world; go to the kang to know your mother. Let’s get down on the kang and learn about shoes!

But if you don’t know why? Okay then, some of me will say it, and some of you will see it.

In fact, the method of diagnosis (debugging) is similar to that of traditional Chinese medicine. You can choose one or two of the four methods of looking, smelling, asking, and palpating to find the answer. .

Hope

There is not much code. After reading it for a while, even without my comments, I believe that careful students will find the real file. If there is a discrepancy between the name and the code when it is introduced, then there must be a problem here. Remember the problem, let’s continue

hear

Even if , I can’t smell anything in the code

Ask

Come on, a very important part of software engineering is Communication may not necessarily be with colleagues who encounter bugs. It may be yourself, it may be QA, and of course it may be PM or your boss. You didn't ask the question you wanted to know; he didn't make it clear what he wanted to answer; it's all over. . . .

So what do I want to know? The following two things are more reasonable as the entrance to debugging:

operating system
Run Environment + Version
How did you test, command line or other means

Answer: macOS;

node.js > 8.0; Command linenode test.js 

chet

The exciting and profound moment has arrived, and I’m going to take action. (In order to fully describe the

debug process, I will pretend that I don’t know everything below in advance)

Prepare the computer, completed

Prepare the running environment

node.js > 9.3.0, Completed

Copied the code, completed

Run it, it was amazing, no error was reported, and the running result was what George G said.

In order to prove that I am not blind, I tried again

in test.js require A file that does not exist at allrequire('./nidayede'), run the code.

Fortunately, an error was reported this time

Error: Cannot find module './nidayede', so I didn’t Crazy. That's a real joy.

So there is the first question

Why can Gouri’s module name still be loaded despite the wrong case?

Is it related to the operating system? Let's try again on windows. Sure enough, we got to windows , the case issue is a problem, Error: Cannot find module './Parent' .

So what is macOS doing? Can't you tell the difference between uppercase and lowercase? So hurry upgoogle (Don’t ask me why not baidu)

The story between node.js and macOS

It turns out that the awesome OS X uses the case-insensitive file by default. system (detailed documentation).

but why? What is the purpose of such an anti-human design?

The story between node.js and macOS

More explanations, come and go

So, this is the reason why you don’t report an error ? (blaming node.js) But that’s the whole truth.

But the matter is not over yet

So what the hell is it like to have a thief as your father?

I have vaguely heard of it. Is there any cache in node.js? Is it caused by that thing? So in the mood to give it a try, I put const ParentIncorrect = require('./Parent') and const Parent = require('./parent') I changed the position and thought, would it be right to load it with the correct name first?

Sure enough, is still wrong. You can't really solve the problem by guessing and pretending

NaBibiParentIncorrect and## Where is #Parent? So I wrote console.log(ParentIncorrect === Parent), and the result was false. So they are really not the same thing, so the problem may be in the introduction part?

So the idea of pretending to look at the source code of node.js was born (in fact, if you don’t look at it, the problem is ultimately I can also figure it out). After a long day, with a feeling of trepidation, I finally clonenode.js Source code (it took a long time, so slow)

Come, let’s enter the mysterious node.js Source code world. Since our question is about require, let's start with her, but find require The definition process requires some patience. I won’t go into details here. I’ll just talk about the search sequence.

##src/node_main.cc => src/node. cc => lib/internal/bootstrap_node.js => lib/module.js 

Found it, this is it

lib/ module.js, get to the point:

lib/module.js => require

<span style="font-size: 14px;">Module.prototype.require = function(path) {<br/>  assert(path, &#39;missing path&#39;);<br/>  assert(typeof path === &#39;string&#39;, &#39;path must be a string&#39;);<br/>  return Module._load(path, this, /* isMain */ false);<br/>};<br/></span>

好像没什么卵用，对不对？她就调用了另一个方法 _load ，永不放弃，继续

lib/module.js => _load

<span style="font-size: 14px;">Module._load = function(request, parent, isMain) {<br/>  //debug代码，么卵用，跳过<br/>  if (parent) {<br/>    debug(&#39;Module._load REQUEST %s parent: %s&#39;, request, parent.id);<br/>  }<br/><br/>  if (isMain && experimentalModules) {<br/>    //...<br/>    //...<br/>    //这段是给ES module用的，不看了啊<br/>  }<br/><br/>  //获取模块的完整路径<br/>  var filename = Module._resolveFilename(request, parent, isMain);<br/><br/>  //缓存在这里啊？好激动有没有？！？终于见到她老人家了<br/>  //原来这是这样的，简单的一批，毫无神秘感啊有木有<br/>  var cachedModule = Module._cache[filename];<br/>  if (cachedModule) {<br/>    updateChildren(parent, cachedModule, true);<br/>    return cachedModule.exports;<br/>  }<br/><br/>  //加载native但非内部module的，不看<br/>  if (NativeModule.nonInternalExists(filename)) {<br/>    debug(&#39;load native module %s&#39;, request);<br/>    return NativeModule.require(filename);<br/>  }<br/><br/>  //构造全新Module实例了<br/>  var module = new Module(filename, parent);<br/><br/>  if (isMain) {<br/>    process.mainModule = module;<br/>    module.id = &#39;.&#39;;<br/>  }<br/><br/>  //先把实例引用加缓存里<br/>  Module._cache[filename] = module;<br/><br/>  //尝试加载模块了<br/>  tryModuleLoad(module, filename);<br/><br/>  return module.exports;<br/>};<br/></span>

似乎到这里差不多了，不过我们再深入看看 tryModuleLoad

lib/module.js => tryModuleLoad

<span style="font-size: 14px;">function tryModuleLoad(module, filename) {<br/>  var threw = true;<br/>  try {<br/>    //加载模块<br/>    module.load(filename);<br/>    threw = false;<br/>  } finally {<br/>    //要是加载失败，从缓存里删除<br/>    if (threw) {<br/>      delete Module._cache[filename];<br/>    }<br/>  }<br/>}<br/></span>

接下来就是真正的 load 了，要不我们先停一停？

好了，分析问题的关键在于不忘初心，虽然到目前为止我们前进的比较顺利，也很爽对不对？。但我们的此行的目的并不是爽，好像是有个什么疑惑哦！于是，我们再次梳理下问题：

son.js 里用首字母大写(不正确)的模块名引用了 parent.js
test.js 里，引用了两次 parent.js ，一次用完全一致的模块名；一次用首字母大写的模块名。结果发现 son instanceof require('./parent') === false

既然没报错的问题前面已经解决了，那么，现在看起来就是加载模块这个部分可能出问题了，那么问题到底是什么？我们怎么验证呢？

这个时候我看到了这么一句话 var cachedModule = Module._cache[filename]; ，文件名是作为缓存的 key ，来吧，是时候看看 Module._cache 里存的模块 key 都是什么牛鬼蛇神了，打出来看看吧，于是我在 test.js 里最后面加了一句 console.log(Object.keys(require.cache)) ，我们看看打出了什么结果

<span style="font-size: 14px;">false<br/>true<br/>[ &#39;/Users/admin/codes/test/index.js&#39;,<br/>  &#39;/Users/admin/codes/test/Parent.js&#39;,<br/>  &#39;/Users/admin/codes/test/parent.js&#39;,<br/>  &#39;/Users/admin/codes/test/son.js&#39; ]<br/></span>

真相已经呼之欲出了， Module._cache 里真的出现了两个 [p|P]arent （ macOS 默认不区分大小写，所以她找到的其实是同一个文件；但 node.js 当真了，一看文件名不一样，就当成不同模块了），所以最后问题的关键就在于 son.js 里到底引用时用了哪个名字（上面我们用了首字母大写的 require('./Parent.js') ），这才导致了 test.js 认贼作父的梗。

如果我们改改 son.js ，把引用换成 require('./parEND.js') ，再次执行下 test.js 看看结果如何呢？

<span style="font-size: 14px;">false<br/>false<br/>[ &#39;/Users/haozuo/codes/test/index.js&#39;,<br/>  &#39;/Users/haozuo/codes/test/Parent.js&#39;,<br/>  &#39;/Users/haozuo/codes/test/parent.js&#39;,<br/>  &#39;/Users/haozuo/codes/test/son.js&#39;,<br/>  &#39;/Users/haozuo/codes/test/parENT.js&#39; ]<br/></span>

没有认贼作父了对不对？再看 Module._cache 里，原来是 parENT.js 也被当成一个单独的模块了。

So, assuming your module file name has n characters, theoretically, in  macOS In a case-insensitive file system, you can node.js maximize it2 of n power caches to

Isn’t it miserable! ? Fortunately macOS You can still change it to case-sensitive, reinstall the system from a grid disk, or create a new partition.

Although the problem is not difficult, the determination and ideas to explore the problem are still important.

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The story between node.js and macOS

<span style="font-size: 14px;">parent.js</span>

<span style="font-size: 14px;">son.js</span>

<span style="font-size: 14px;">test.js</span>

Why can Gouri’s module name still be loaded despite the wrong case?

So what the hell is it like to have a thief as your father?

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