How to use the 'main' parameter in package.json?

From the npm documentation:

The main field is a module ID that is the primary entry point to your program. That is, if your package is named foo, and a user installs it, and then does require("foo"), then your main module's exports object will be returned.

This should be a module ID relative to the root of your package folder.

For most modules, it makes the most sense to have a main script and often not much else.

To put it short:

1. You only need a main parameter in your package.json if the entry point to your package differs from index.js in its root folder. For example, people often put the entry point to lib/index.js or lib/<packagename>.js, in this case the corresponding script must be described as main in package.json.
2. You can't have two scripts as main, simply because the entry point require('yourpackagename') must be defined unambiguously.

To answer your first question, the way you load a module is depending on the module entry point and the main parameter of the package.json.

Let's say you have the following file structure:

my-npm-module
|-- lib
|   |-- module.js
|-- package.json

Without main parameter in the package.json, you have to load the module by giving the module entry point: require('my-npm-module/lib/module.js').

If you set the package.json main parameter as follows "main": "lib/module.js", you will be able to load the module this way: require('my-npm-module').