What *REALLY* happens when we require() a module in Nodejs?

modules.md

What happens when we require() a module?

There are 5 steps in this process:

• Resolving & Loading.
• Wrapping.
• Execution.
• Returning Exports.
• Caching.

Step 1: Resolving & Loading:

Before explaining this step, You should know that there are 3 types of modules:

• Core modules such as (fs, http, etc..)

• Your modules such as (./utils/someUtil.js)

• 3rd party modules such as (express)

• Process:

  • Start with core modules.
  • If the module is begin with a relative path such as ./ or ../, It will try to load Your modules.
  • If there's no file, It will try to see if it's a folder and load the index.js from it.
  • else, It will try to go node_modules and try to find it there, might be a 3rd party module.
  • otherwise, It will raise an error.

Step 2: Wrapping:

• Our module gets wrapped into an Immediately invoked function expression Aka IIFE which expose objects to use like exports, require, module, __filename and __dirname

• This will achieve 2 things:

  • Get access to those exposing objects.
  • Keeps our Top-level code variables private, because They will be scoped only to the current module.

• (function (exports, require, module, __filename, __dirname) {
    // Module's Implementation Code
  })();

Step 3: Execution:

• Here, the execution of our module starts to happen.

Step 4: Returning Exports:

• Here, Any exports are done by either way (module.exprots or exports.someFunc) will be returned to be required again in other modules

Step 4: Caching:

• The module is cached after the first time is loaded.
• The code inside the module is executed only once.
• In subsequent calls, the result will be retrieved from the cache.