import - What does the require npm install module in node.js do?

Question

I just discovered that the require module will also affect subsequent require, taking moment and moment-timezone as examples, that is to say:

/app.js

var moment = require('moment');
console.log(moment.tz()); 
// 这样会报错，因为moment.tz()是moment-timezone这个module才有的function
/*
TypeError: moment.tz is not a function
*/

But if I require moment-timezone first, and then require moment, npm install moment, I can still require('moment') and this moment can use the function of moment-timezone, as follows

/app.js

require('moment-timezone')
var moment = require('moment');
console.log(moment.tz()); //moment.utc("2017-06-27T06:59:14.475+00:00")

What I don't understand is why after require('moment-timezone'), even if I don't have npm install moment, I can still require('moment'); without reporting an error?
What is the design of such node modules or is there any special term that can cause such behavior?

By the way, if the above moment-timezone can be designed like this, suppose I released a module called noname for people to use on npm today. As long as the program executes require('noname'); first, is it possible? Changed the return content of the subsequent require('express') module to achieve a similar effect to the following:

require('noname');
var express = require('express');
express.thisIsMyExpress();//这是被我换过的express module

Answer 1

1. Why after require('moment-timezone'), even if I don't npm install moment I can still require('moment') without getting an error?

View moment-timezoneDependencies

$> npm info moment-timezone dependencies 
{ moment: '>= 2.9.0' }

It can be seen that moment-timezone is dependent on moment, which means that moment will be automatically installed when installing moment-timezone, so it can be used without installing it separately.

2. Why does require('moment-timezone') affect the subsequent assignment of ｀moment` in var moment = require('moment')

View moment-timezonesource code

  1 //! moment-timezone.js
  2 //! version : 0.5.13
  3 //! Copyright (c) JS Foundation and other contributors
  4 //! license : MIT          
  5 //! github.com/moment/moment-timezone
  6 
  7 (function (root, factory) {
  8   "use strict";            
  9 
 10   /*global define*/        
 11   if (typeof define === 'function' && define.amd) {
 12     define(['moment'], factory);                 // AMD
 13   } else if (typeof module === 'object' && module.exports) {
 14     module.exports = factory(require('moment')); // Node
 15   } else {
 16     factory(root.moment);                        // Browser                  
 17   }
 18 }(this, function (moment) {
 19   "use strict";

On line 14, you can see that moment-timezone has modified require('moment'). As we all know, the npm module will be cached, so the subsequent var moment = require('moment') will be affected

By the way, directly modifying modules is the same as modifying global variables, which is not a good practice. The moment-timezone module here is basically a patch of the ｀moment｀ module and is a special case.