Ifdef

A common pattern in Node is to store singletons within a Module's closure. This is an extremely powerful pattern, and its simplicity leaves little to be desired. However, it's not uncommon for this pattern to break when:

• The require cache goes bad (happens with some modules, but infrequent).
• Multiple versions of the same module are required (way more frequent).

In this case, it'd be great to have a Node equivalent of #ifdef. That's what this module is for.

Installation

npm install ifdef --save

Usage

From within your singleton module code, you need five lines of code. The first line is your standard require statement:

var ifdef = require('ifdef')

The second three are a check against existing guards, returning the guarded value if necessary. Here, GUARD_TERM should be a term unique to your module. The name provided in package.json is unique across modules, and should be considered. If you're worried about collision, don't be afraid to "decorate" it a little, i.e. global_mongoose_connection instead of mongoose:

if (ifdef('GUARD_TERM')) {
  return module.exports = ifdef('GUARD_TERM')
}

Following this block, build out your singleton. The third block of ifdef code, then, assigns this singeton as the guarded value. If you have not already assigned it to module.exports:

module.exports = ifdef('GUARD_TERM', SINGLETON)

Notice that ifdef just returns the guarded term, making this a one-liner. If you've already assigned what you want in module.exports, it's even simpler:

ifdef('GUARD_TERM', module.exports)

Alternatives

As far as I know, there aren't any independent of writing your own guard. If I'm wrong, please let me know and I'll add them here. As far as writing your own is concerned, here's what it looks like:

if (global.__my_module_name) {
  return module.exports = global.__my_module_name
}
 
// Initialization here
 
module.exports = global.__my_module_name = SINGLETON