How to run Node.js module initialization properly

Question

In a Node.js module, I need to run some initialization code, some code to be run only once, to parse a YAML config file on my application startup.

As a Node.js novice, I noticed that modules are loaded once, and only once (and that's very fine like that). I mean:

// ./mymodule.js

console.log('mymodule.js was run');

... displays the given message only once, even if ./mymodule.js was needed in multiple loaded modules. And that's fine.

So, for my init code, I can imagine to implement it simply like that:

// ./mymodule.js

function mymodule_init()
    // Parse my config file
    // ...
    // Done
}

// Run my init:
mymodule_init();

console.log('mymodule.js was run');

But is it the right way to run my initialization code? Isn't there a more regular way to do it, like subscribing to a kind of 'module_init' event, which would guarantee that my init code would be run once and only once?

Note: I found this SO link about the question : Initialize nodejs module once, use many times, but there's no really frank answer about it.

saille · Accepted Answer · 2016-11-03 01:55:19Z

6

Node.js documention on the subject of module caching states:

Multiple calls to require('foo') may not cause the module code to be executed multiple times. This is an important feature. With it, "partially done" objects can be returned, thus allowing transitive dependencies to be loaded even when they would cause cycles.

So I don't see anything wrong with initialization code executed directly in a module. I am doing this in several Node.js apps, as you indicate, like so:

// ./mymodule.js

function mymodule_init()
    // Parse my config file
    // ...
    // Done
}

// Run my init:
mymodule_init();

console.log('mymodule.js was run');

answered Nov 3, 2016 at 1:55

saille

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Comments

yolenoyer · Accepted Answer · 2016-11-02 22:26:20Z

5

Your question will mostly have opinionated answers, but anyway.

I usually use a singleton class for this kind of work.

Something like

// config.js

function Config() {
   this.config = parseYAML("./path/to/yaml-file.yaml");   
}

Config.prototype.get = function( property ) {
   return this.config[property];
}

module.exports = new Config();

And in your app or modules

// app.js

const config = require( "./config.js" );

console.log( config.get("some-config-property") );

Most of the modules either use this or an anonymous function, which is called immediately e.g.

module.exports = (function() {
   return parseYAML("./path/to/yaml-file.yaml");
})();

Both cases you will do it right, so it actually doesn't matter so much, as long as your code is readable.

edited Nov 2, 2016 at 22:26

yolenoyer

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answered Nov 2, 2016 at 22:07

Ivan Drinchev

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1 Comment

yolenoyer Over a year ago

Your answer is working well and is better written than my example, but it doesn't really respond to the question, I mean, can I formally trust the Node.js cache to run this code only once?

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