Learn Node.js from me (4)---Module loading method and mechanism of Node.js_javascript skills

Others, such as third-party modules or local modules installed through NPM, each module will expose a public API. So that developers can import it. Such as

Copy code The code is as follows:

var mod = require('module_name')

After this sentence is executed, Node will load built-in modules or modules installed through NPM. The require function returns an object. The API exposed by the object may be a function, object, or attribute such as a function, array, or even any type of JS object.

Here is the loading and caching mechanism of the node module

1) Load the built-in module (A Core Module)
2) Load the file module (A File Module)
3) Load the file directory module (A Folder Module)
4) Load Modules in node_modules
5) Automatically cache loaded modules

1. Load built-in modules

Node’s built-in modules are compiled into binary form, and are referenced directly by name instead of file path. When a third-party module has the same name as a built-in module, the built-in module will overwrite the third-party module with the same name. Therefore, when naming, you need to be careful not to have the same name as the built-in module. Such as getting an http module

Copy code The code is as follows:

var http = require('http')

The returned http is the built-in module of Node that implements the HTTP function.

2. Load file module

Absolute path

Copy code The code is as follows:

var myMod = require('/home/base/my_mod ')

or relative path

Copy code The code is as follows:

var myMod = require('./my_mod')

Note that the extension ".js" is ignored here, the following is equivalent

Copy code The code is as follows:

var myMod = require('./my_mod')
var myMod = require('./my_mod.js')

　
3. Load the file directory module

You can directly require a directory, assuming there is a directory named folder, such as

Copy code The code is as follows:

var myMod = require('./folder')

At this point, Node will search the entire folder directory. Node will assume that the folder is a package and try to find the package definition file package.json. If the folder directory does not contain the package.json file, Node will assume that the default main file is index.js, which will load index.js. If index.js doesn't exist either, then loading will fail.

Suppose the directory structure is as follows

package.json is defined as follows

Copy code The code is as follows:

{
"name": "pack",
"main": "modA.js"
}

At this point require('./folder') will return the module modA.js. If package.json does not exist, the module index.js will be returned. If index.js also does not exist, a loading exception will occur.

4. Load the modules in node_modules

If the module name is not a path or a built-in module, Node will try to search in the node_modules folder of the current directory. If the node_modules in the current directory is not found, Node will search from the node_modules in the parent directory and recurse until the root directory.

Don’t worry, the npm command allows us to easily install, uninstall, and update the node_modules directory.

5. Automatic caching of loaded modules

The loaded module Node will be cached without having to search again every time. Here is an example

modA.js

Copy code The code is as follows:

console.log('Module modA starts loading...')
exports = function() {
console.log('Hi')
}
console.log ('Module modA loaded')

init.js

Copy code The code is as follows:

var mod1 = require('./modA')
var mod2 = require('./modA')
console.log(mod1 === mod2)

Command line execution:

node init.js

Enter as follows

You can see that although require is executed twice, modA.js is still only executed once. mod1 and mod2 are the same, that is, both references point to the same module object.