The principle, usage and some common application scenarios of Node.js module system

Node.js is an open source, cross-platform backend programming environment based on the JavaScript runtime. Node.js' module system is one of its core features, which provides a convenient and modular way to organize and reuse code. This article will introduce the principle, usage and some common application scenarios of the Node.js module system in detail.

The role of the module system

The main function of the module system is to encapsulate codes with similar functions in a separate file, so that the code structure is clearer and the modules are isolated from each other. It can also help us improve code maintainability and reusability, reducing naming conflicts and global variable problems.

Classification of modules

In Node.js, modules can be divided into two categories: core modules and file modules.

core module

The core module is an official module provided by Node.js, which can be requireimported and used directly through the method without prior installation. For example, httpmodules for creating servers and handling HTTP requests, fsmodules for file operations, and so on.

file module

The file module is a module written by ourselves and requireimported through the method. A file module can be a JavaScript file ( .js), a JSON file ( .json), or a compiled C++ extension ( .node).

Module export and import

In Node.js, the content of a module can module.exportsbe exported through the object, and other modules can requireimport the content through the method.

export

We can assign the content that needs to be exported to module.exportsthe properties or methods of the object. For example, we could math.jsdefine the following in a file called :

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

function subtract(a, b) {
    
    
  return a - b;
}

module.exports = {
    
    
  add,
  subtract
};

import

To use the exported content in another module, we can use requirethe method and assign it to a variable. For example, in another file:

const math = require('./math.js');

console.log(math.add(1, 2)); // 输出：3
console.log(math.subtract(5, 3)); // 输出：2

Module lookup rules

When using requirethe method to import a module, Node.js will search for the module file according to certain search rules.

core module

For core modules, Node.js will first look in the list of built-in core modules. If the corresponding core module is found, the module is returned directly.

file module

For file modules, Node.js looks in the following order:

1. If the import is a path (beginning with ./or ../), it will be treated as a relative path, and the module file will be found in the directory where the current file is located.
2. If the import is an absolute path (beginning with /), it will be treated as an absolute path, and the module file will be looked for under this path.
3. node_modulesIf the import is not a path, Node.js will search for modules in all levels of folders according to certain rules . node_modulesThe specific rule is to start from the directory where the current file is located, search for folders in the upper directory in turn , and reach the root directory. In each node_modulesfolder, the corresponding module file will be searched according to the imported module name.

module cache

Node.js caches loaded modules to improve performance. This means that if a module is loaded multiple times, it will only be executed once and return the same object.

Circular dependencies of modules

When two modules depend on each other, it may cause the problem of circular dependency. For example, module A depends on module B, and module B depends on module A. In order to avoid the problem of circular dependencies, Node.js will first load the first-level dependencies of modules during execution, and then load other dependencies layer by layer.

Application scenarios of the module system

The module system of Node.js can be applied in many scenarios, the following are some common application scenarios:

Build a web server

Using the module system can easily organize and reuse code, and build a well-structured and extensible Web server.

database operation

By encapsulating database operations into modules, these modules can be reused in different places. For example, use the same database query logic in different route handlers.

file operation

File operation is a common application scenario of Node.js. By modularizing, we can better organize and reuse file-related code.

Tool library development

By encapsulating commonly used tool functions into modules, these functions can be easily reused in multiple projects and development efficiency can be improved.

Summarize

Node.js' module system is one of its core features, which provides a convenient, modular and reusable way of organizing code. By importing and exporting modules, we can encapsulate the code of related functions together to improve the maintainability and reusability of the code. At the same time, the module system of Node.js also has features such as finding rules, caching, and resolving circular dependencies. Mastering the principles and usage of the Node.js module system is very important for developing efficient and maintainable Node.js applications.