How to use nodejs module

Node.js is an open source cross-platform JavaScript runtime environment based on event-driven, asynchronous I/O. In Node.js, modules are used to organize code. Each module is an independent file that can be called and logically reused between modules through import/export. This article will introduce the basic usage of Node.js modules.

1. Import modules

In Node.js, use the require() function to import modules. The parameter of the require() function is the name of the module that needs to be imported. The module name can be a relative path or an absolute path, or it can be a system module.

const module1 = require('./module1')
const module2 = require('./module2')
const fs = require('fs')

2. Export module

In Node.js, use the module.exports object or the exports object to export a module. The default value of module.exports and exports is an empty object {}, but module.exports can be reassigned, while exports is just a reference to module.exports and cannot be reassigned.

function add(a, b) {
  return a + b
}

module.exports = {
  add: add
}

exports.sub = function(a, b) {
  return a - b
}

3. Module cache

When loading a module, Node.js will cache the loaded module to improve operating efficiency. When the same module is loaded a second time, Node.js reads directly from the cache instead of reloading.

4. Module path resolution sequence

If the module name specified in the require() function is not a system module, the module path will be parsed in the following order:

1. If the module with the specified name is a core module, subsequent path analysis will be skipped and the module will be returned directly;

2. If the module with the specified name starts with './' or '../', then Locate module files according to path resolution rules. The path resolution rule is: if the directory containing the module is the current directory, search for the specified file in the directory. If the directory containing the module is one that has been added to the $NODE_PATH environment variable, the specified file is found in that directory. If the directory containing the module is not in the above two directories, the specified file will be searched in the current directory and the directory that has been added to the $NODE_PATH environment variable;

3. If the module with the specified name is not a core module , nor a file name starting with './' or '../', then the module files are searched in the following order:

1.在包含该模块的目录下查找 node_modules 目录；
2.在当前目录下查找 node_modules 目录；
3.在已添加到 $NODE_PATH 环境变量的目录中查找 node_modules 目录；

4. If the specified module file is still not found, throw mistake.

5. Circular references of modules

In Node.js, circular references between modules are allowed, but circular references may cause undefined modules. In order to avoid circular references of modules, you need to pay attention to the way you use exports and the timing of imports.

6. Summary

This article introduces the basic usage of Node.js modules, including importing modules, exporting modules, module caching, module path resolution order and circular references of modules, etc. Understanding these contents will help us better understand and use the idea of ​​Node.js modular programming.

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