How to pass variables into NodeJS modules?

Question

In one of my JS files I include another one. How can I set variables in the included module?

I thought doing something like this would work

var mymodule = require('mymodule.js');
mymodule.myvariable = 'test';

And then in mymodule

this.myvariable === 'test';

But this doesn't work, it's undefined. What are the various options for passing a value into a module? I could just add the variable as a parameter to every function I call in mymodule, but that isn't ideal.

Is there a way to do it without globals, so that I can set the variables independently in various required modules, like this?

var mymodule1 = require('mymodule.js');
var mymodule2 = require('mymodule.js');
mymodule1.myvariable = 'test1';
mymodule2.myvariable = 'test2';

Answer 1

The problem with what you were doing is that you set the variable after importing, but this.myvariable === 'test'; was being called when the module was imported, before your variable was set.

You can have your module export a function and then call the function when you import, passing your variable as an argument.

module.exports = function(myVar) {
  var myModule = {
    // has access to myVar 
    ...
  };

  return myModule; 
};

When you import,

var myModule = require('myModule')(myVar);

If you use this method, keep in mind that you get a different instance of your module wherever you import, which may not be what you want.

If you want to set values of a module from outside the module, a good option is to have your module export an object with a setter method, and use that to set the value of the variable as a property of the object. This makes it more clear that you want this value to be settable, whereas just doing myModule.myVar = can set you up for confusion later.

module.exports = {
  myVar: null,

  setMyVar: function(myVar) {
    this.myVar = myVar;
  },

  ...

};

In this case you're accessing the same instance of the model wherever you import it.

Edit in response to comment

In the first option you show where you get a different instance each time, how can I export multiple functions that each share the same myVar? If that module exports 5 functions each that need myVar, can I set it in one place like at import time rather than passing it into each function?

Not entirely sure if I understand what you're describing, but you could do something like this:

module.exports = function(myVar) {
  var modules = {};
  modules.someModule = {...};
  modules.anotherModule = {...};
  ...

  return modules;
};

Each of these sub-modules would have access to the same myVar. So you would import as above and the result would be an object containing each of your five modules as properties. I can't say whether this is a good idea, it's getting pretty convoluted, but maybe it makes sense for your situation.