Watch out for globals in node.js modules!

Globals, or global variables are known to be risky.
However using the ‘var’ keyword should ensure file level definition.

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As such shouldn’t it be safe to use module level variables?

The answer is no, and it should be avoided at all costs.

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Why module level variables are bad?

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Node require will wrap your module with a function as follows:

Copy‍~ $ node

require('module').wrapper
[ '(function (exports, require, module, __filename, __dirname) { ',
'\n});' ]

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The calling node will assign to these arguments when it will invoke the wrapper function.

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This is what makes them look as if they are globals in the scope of your node module.

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It seems we have globals in our module however:

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- export is defined as a reference to module.exports prior to that.
– require and module, are defined by the function executed.
– __filename and __dirname are the filename and folder of your current module.

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Caching – a double edge sword

Node will then cache this module, so the next time you require the file, you won’t actually get a fresh copy, but you’ll be getting the same object as before.

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This means you’ll be using the same global modules variables in multiple places, which means danger!

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Here is a code example that illustrated the problem:

Copy‍//moduletest.js
'use strict';
var x = 0;
 
module.exports = function (val) {
  console.log(`val : ${val}, x: ${x}`);
  if (val !== x && x !== 0) throw new Error(`failure!!! ${x} != ${val}`);
  x = val;
}
 
//main.js
const fn1 = require('./moduletest');
const fn2 = require('./moduletest');
setInterval(function () {
    fn1('a');
  },200
);
 
setInterval(function () {
    fn2('b');
  },50
);

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I’m running here two calls to the same function, with a small delay between each call, after a few runs we will notice that the function will run over each others variables. Which is an example of a module global issue.

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How to solve globals?

There are multiple potential solutions to this global issue, I’ll present you with two potential solutions

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Solution 1 – Functional

If we define a local scope inside our module, we can return a new set of variables for each run.

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We will use a ‘let’ keyword, along with a scoped function (not needed, but nicer and better scope control).

Copy‍//testmodule.js 
'use strict';
module.exports = (function() {
  let x = 0;
 
  return function (val) {
    console.log(`val : ${val}, x: ${x}`);
    if (val !== x && x !== 0) 
      throw new Error(`failure!!! ${x} != ${val}`);
    x = val;
  }
});
//main.js
fn1 = require('./testmodule')(); //<--- calling a function each time
fn2 = require('./testmodule')();
 
// fn1 and fn2 are new functions with new variables, we busted the cache !! :)
// notice I also use let, to ensure scope variables, and not hoisted vars.

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Solution 2 – use Classes

We can just define a class then create a new class for each run.

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This way each variable is a private member of that class, ensuring proper encapsulation.

Copy‍//testmoduleclass.js
'use strict';
 
class FunctionRunner {
  constructor() {
    this.x = 0;
  }
 
  fn(val) {
    console.log(`val : ${val}, x: ${this.x}`);
    if (val !== this.x && this.x !== 0) throw new Error(`failure!!! ${this.x} != ${val}`);
    this.x = val;
  }
}
 
module.export = FunctionRunner;
 
//testclass.js
const FunctionRunner = require('./testmoduleclass.js');
 
const fn1 = new FunctionRunner();
const fn2 = new FunctionRunner();
// now each fn holds it's own set of variables.
// no risk at all :)

For complete code have look at this repository:
https://github.com/CoreTeamIO/globals-in-node-modules

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