NodeJs Require機制

寫NodeJS程序的時候，經常需要寫require，但是對於require的原理又不是很了解，去看了一下官方文檔，這里做下筆記以免遺忘！

require(X) from module at path Y
1. If X is a core module,
   a. return the core module
   b. STOP
2. If X begins with './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
3. LOAD_NODE_MODULES(X, dirname(Y))
4. THROW "not found"

LOAD_AS_FILE(X)
1. If X is a file, load X as JavaScript text.  STOP
2. If X.js is a file, load X.js as JavaScript text.  STOP
3. If X.json is a file, parse X.json to a JavaScript Object.  STOP
4. If X.node is a file, load X.node as binary addon.  STOP

LOAD_AS_DIRECTORY(X)
1. If X/package.json is a file,
   a. Parse X/package.json, and look for "main" field.
   b. let M = X + (json main field)
   c. LOAD_AS_FILE(M)
2. If X/index.js is a file, load X/index.js as JavaScript text.  STOP
3. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
4. If X/index.node is a file, load X/index.node as binary addon.  STOP

LOAD_NODE_MODULES(X, START)
1. let DIRS=NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
   a. LOAD_AS_FILE(DIR/X)
   b. LOAD_AS_DIRECTORY(DIR/X)

NODE_MODULES_PATHS(START)
1. let PARTS = path split(START)
2. let I = count of PARTS - 1
3. let DIRS = []
4. while I >= 0,
   a. if PARTS[I] = "node_modules" CONTINUE
   b. DIR = path join(PARTS[0 .. I] + "node_modules")
   c. DIRS = DIRS + DIR
   d. let I = I - 1
5. return DIRS

　

總結一下偽代碼就是：

一般我們在nodjejs中require的時候通常有三種寫法。

1) require('內置module') -> 直接返回core module

2) require('./或者../這種帶有路徑的') -->先查找有沒有這個文件,沒有然后當作文件夾來查找

3) require('包名比如jquery這種的的') --> 去當前目錄同級的node_modules目錄下查找，拿jquery舉例，先查找node_modules/jquery.js是否存在，不存在則查找node_modules/jquery目錄是否存在，如果存在，那么查找node_modules/jquery/package.json中的main字段指向，然后找到相應的js文件然后進行加載。