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With RequireJS on the front-end, we can listen to see when modules get loaded into the runtime module cache using:

requirejs.onResourceLoad = function (context, map, depArray) {
    console.log('onResourceLoad>>>', 'map.id:', map.id, 'context:', context);
};

Can we do this with Node.js somehow? Will be useful for debugging. Especially when servers are loading different files (or in different order) based on configuration.

I assume this might be documented in

.html

but I am not seeing anything

With RequireJS on the front-end, we can listen to see when modules get loaded into the runtime module cache using:

requirejs.onResourceLoad = function (context, map, depArray) {
    console.log('onResourceLoad>>>', 'map.id:', map.id, 'context:', context);
};

Can we do this with Node.js somehow? Will be useful for debugging. Especially when servers are loading different files (or in different order) based on configuration.

I assume this might be documented in

https://nodejs/api/modules.html

but I am not seeing anything

• javascript
• node.js

Share Improve this question Follow edited Oct 15, 2016 at 0:20 Alexander Mills asked Oct 14, 2016 at 23:32 Alexander MillsAlexander Mills 100k166166 gold badges537537 silver badges916916 bronze badges

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3 Answers 3

Sorted by: Reset to default 8

If you look at the source code for require(), you will find this:

Module._load = function(request, parent, isMain) {
    if (parent) {
        debug('Module._load REQUEST %s parent: %s', request, parent.id);
    }

This shows that you can leverage the debug() call to get the information you need. In order to do this, you will notice that module is setup using util.debuglog('module'). This means that you need to run your application with with the NODE_DEBUG variable set to module. You can do it the following way from the console:

NODE_DEBUG=module node main.js

This will log what you are looking for.

I'm not aware of a documented callback API for the purpose of module load callbacks (although a logging mechanism for module loading appears to exist).

Here's a quick workaround to the apparent lack of such a callback, by monkeypatching Module._load:

const Module = require('module');
const originalModuleLoad = Module._load;
Module._load = function() {
    originalModuleLoad.apply(this, arguments);
    console.log("Loaded with arguments " + JSON.stringify(arguments));
}

I executed the above code in a REPL and then did require('assert'). Lo and behold:

> require('assert')
Loading with arguments {"0":"assert","1":{"id":"<repl>","exports":{},"filename":null,"loaded":false,"children":[],"paths":["/Users/mz2/Projects/manuscripts-endnote-promo/repl/node_modules","/Users/mz2/Projects/manuscripts-endnote-promo/node_modules","/Users/mz2/Projects/node_modules","/Users/mz2/node_modules","/Users/node_modules","/Users/mz2/.nvm-fish/v6.1.0/lib/node_modules","/Users/mz2/.node_modules","/Users/mz2/.node_libraries","/Users/mz2/.nvm-fish/v6.1.0/lib/node"]},"2":false}

Please don't think about using code like above for anything but debug only purposes.

Because node.js modules are imported (required) synchronously, simply having the require statement means the module is imported.

While RequireJS can import modules asynchronously, the even listening is an important feature, but native require in Node.js leaves this necessity out. This way, as you probably know:

const module = require('module')
// You can use the module here, async or sync.

To add to that, not only require is sync, but also in order to use a module it has to be explicitly required in the same file where it's used. This can be bypassed in several ways, but best practice is to require in every module where you use a module.

For specific modules which require async initialization, either the module should provide an event, or you can wrap the init function using a promise or a callback. For example, using a promise:

const module = require('module')
// Create a promise to initialize the module inside it:
const initialized = new Promise((resolve, reject) => {
    // Init module inside the promise:
    module.init((error) => {
        if(error){
            return reject(error)
        }
        // Resolve will indicate successful init:
        resolve()
    })
})

// Now with wrapped init, proceed when done:
initialized
    .then(() => {
        // Module is initialized, do what you need.
    })
    .catch(error => {
        // Handle init error.
    })

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