The js_of_ocaml documentation says about making a Node.js module:
Js.export and Js.export_all will export a value to module.exports if it exists.
What I'm confused about is the "if it exists" part. How do I make sure that the module def exists? i.e. if I'm making an OCaml library that I'd like to be able to import from JavaScript (TypeScript actually), how can I make sure that I get such a module as the output of js_of_ocaml?
If module.exports exists, a value is exported as node module.
And if module.exports doesn't exists (on web browser for example), a value is exported as global variable.
Related
Does the node interpreter look for core modules (let's say "fs") within the node binary? If yes, are these modules packaged as js files. Are the core modules that are referenced within our code converted to c/c++ code first and then executed? For example, I see a method in the _tls_common.js (https://github.com/nodejs/node/blob/master/lib/_tls_common.js) file called "loadPKCS12" and the only place that I see this method being referenced/defined is within the "node_crypto.cc" file (https://github.com/nodejs/node/blob/master/src/node_crypto.cc). So how does node link a method in javascript with the one defined in the c/c++ file?
here is the extract from the _tls_common.js file that makes use of the "loadPKCS12" method:
if (passphrase) {
c.context.loadPKCS12(buf, toBuf(passphrase));
} else {
c.context.loadPKCS12(buf);
}
}
} else {
const buf = toBuf(options.pfx);
const passphrase = options.passphrase;
if (passphrase) {
c.context.loadPKCS12(buf, toBuf(passphrase));
} else {
c.context.loadPKCS12(buf);
There are two different (but seemingly related) questions asked here. The first one is: "How the core modules work?". Second one being "How does NodeJS let c++ code get referenced and executed in JavaScript?". Let's take them one by one.
How the core modules work?
The core modules are packaged with NodeJS binary. And, while they are packaged with the binary, they are not converted to c++ code before packaging. The internal modules are loaded into memory during bootstrap of the node process. When a program executes, lets say require('fs'), the require function simply returns the already loaded module from cache. The actual loading of the internal module obviously happens in c++ code.
How does NodeJS let c++ code get referenced in JS?
This ability comes partly from V8 engine which exposes the ability to create and manage JS constructs in C++, and partly from NodeJS / LibUV which create a wrapper on top of V8 to provide the execution environment. The documentation about such node modules can be accessed here. As the documentation states, these c++ modules can be used in JS file by requiring them, like any other ordinary JS module.
Your example for use of c++ function in JS (loadPKCS12), however, is more special case of internal c++ functionality of NodeJS. loadPKCS12 is called on a object of SecureContext imported from crypto c++ module. If you follow the link to SecureContext import in _tls_common.js above, you will see that the crypto is not loaded using require(), instead a special (global) method internalBinding is used to obtain the reference. At the last line in node_crypto.cc file, initializer for internal module crypto is registered. Following the chain of initialization, node::crypto::Initialize calls node::crypto::SecureContext::Initialize which creates a function template, assigns the appropriate prototype methods and exports it on target. Eventually these exported functionalities from C++ world are imported and used in JS-World using internalBinding.
Currently I am trying to use TypeScript to create JavaScript-Files which are then required in a index.js file. I am using VS 2015 Update 3 with node.js tools 1.2 RC. Sadly it is not working like I thought it would.
To begin with here is my initial idea:
I have a node module (to be precise, it is a deployd module http://docs.deployd.com/docs/using-modules/). This module is handling payment providers like paypal or stripe. Now I want to use TypeScript to write interfaces, classes and use types to make it easier to add new payment providers. The old .js files should still be there and used. I want to migrate step by step and use the self-written and compiled .js files together. So I thought I can create .ts files, write my code, save, let VS compile to js and require the compiled js file in another js file. Okay, that is my idea... Now the problem
I have a PaymentProvider.ts file which looks like this
import IPaymentProvider = require("./IPaymentProvider.ts"); // Interface, can't be realized in JavaScript, just TypeScript
export abstract class PaymentProvider implements IPaymentProvider.IPaymentProvider
{
providerName: string;
productInternalId: number;
constructor(providerName : string)
{
this.providerName = providerName;
}
//... some methods
};
The other file is PaypalPaymentProvider.ts
import PaymentProvider = require("./PaymentProvider.ts");
export class PaypalPaymentProvider extends PaymentProvider.PaymentProvider
{
constructor()
{
super("paypal");
}
// more methods
}
VS 2015 doesn't show any errors. The js and .js.map files are generated. Now I thought I could require the files and that's it. I tried to use the PaypalPaymentProvider.js like this const PaypalPaymentProvider = require("./lib/payment-provider/PaypalPaymentProvider.js"); (yes, it is located there) but it's not working. When starting the index.js via node I get the following error:
...\Path\PaymentProvider.ts:1 (function (exports, require, module, __filename, __dirname) { import IPaymentProvider = require("./IPaymentProvider.ts"); Unexpected token import....
I find it strange that this is the error, because JavaScript doesnt't have Interfaces. The compiled IPaymentProvider.js is empty.
Also I thought that TypeScript is mainly for development and the compiled JavaScript for production. So why it is requiring a ts-file? I thought imports in typescript will be converted to require of the compiled js-file?
Do I need to require all compiled js files and not only the one I currently try to use? (I don't think so...)
To be honest, I think the main problem is that I am new to TypeScript and make something wrong from the very beginning.
Any help/advice? Thanks!
I have the solution... Thanks to Paelo's links I was able to see that I need to omit the file ending! So the really simple solution was to write
import IPaymentProvider = require("./IPaymentProvider");
instead of
import IPaymentProvider = require("./IPaymentProvider.ts");
When I changed that in every ts file it worked perfectly!
In the guide of the Brunch website, they start the config file as following module.exports = config:, however most of the skeletons you can find on the same website use another syntax exports.config =.
What is the difference between them? Are both javascript CommonJS Module?
I had a look directly at the doc: module node documentation. Something I should have done at first :)
The exports variable that is available within a module starts as a reference to module.exports. As with any variable, if you assign a new value to it, it is no longer bound to the previous value.
If you want the root of your module's export to be a function (such as a constructor) or if you want to export a complete object in one assignment instead of building it one property at a time, assign it to module.exports instead of exports.
Finally, they said:
As a guideline, if the relationship between exports and module.exports seems like magic to you, ignore exports and only use module.exports.
Et voilà!
How can I tell that a script written in node is being called by require?
From the node docs:
Accessing the main module
When a file is run directly from Node, require.main is set to its
module. That means that you can determine whether a file has been run
directly by testing
require.main === module
For a file foo.js, this will be true if run via node foo.js, but false
if run by require('./foo').
Because module provides a filename property (normally equivalent to
__filename), the entry point of the current application can be obtained by checking require.main.filename.
You can check property parent of module object. If your module called by require module.parent will return caller module, otherwise module.parent is null.
I was looking in the node.js module documentation, and noticed that each module has a property- module.parent. I tried to use it, but got burnt by the module caching- module.parent only ever seems to the module that first require()'d it, irrespective of current context.
So what is the usage of it? Is there any other way for me to get a reference to the current require()ing module? Right now I'm wrapping the module in a function, so that it is called like:
require("mylibrary")(module)
but that seems sub-optimal.
The "parent" is the module that caused the script to be interpreted (and cached), if any:
// $ node foo.js
console.log(module.parent); // `null`
// require('./foo')
console.log(module.parent); // `{ ... }`
What you're expecting is the "caller," which Node doesn't retain for you. For that, you'll need the exported function you're currently using to be a closure for the value.
There is a workaround for this. Node adds a module to the module cache before it finishes loading it. This means that a module can delete itself from the module cache while it's loading! Then every time the module is require'd a new instance of the module is loaded.
Magic.js
console.log('Required by ' + module.parent.filename);
delete require.cache[__filename];
Module1.js
//prints "Required by Module1.js"
require('./Magic');
Module2.js
//prints "Required by Module2.js"
require('./Magic');
Of course the side-effect of this is that your module is no longer a singleton, so you have to code Magic.js with that in mind. If you need to store global data you can always keep it in a require()'ed module that doesn't delete itself from the cache.
Update for 2022
Note the technique described above doesn't work for ES Modules included with import. As far as I know there is no good way to detect the importing ES Module.