全部一起

¥All together

要获取调用 require() 时将加载的确切文件名，则使用 require.resolve() 函数。

¥To get the exact filename that will be loaded when require() is called, use the require.resolve() function.

综上所述，这里是 require() 的伪代码高级算法：

¥Putting together all of the above, here is the high-level algorithm in pseudocode of what require() does:

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 '/'
   a. set Y to be the file system root
3. If X begins with './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
   c. THROW "not found"
4. If X begins with '#'
   a. LOAD_PACKAGE_IMPORTS(X, dirname(Y))
5. LOAD_PACKAGE_SELF(X, dirname(Y))
6. LOAD_NODE_MODULES(X, dirname(Y))
7. THROW "not found"

MAYBE_DETECT_AND_LOAD(X)
1. If X parses as a CommonJS module, load X as a CommonJS module. STOP.
2. Else, if the source code of X can be parsed as ECMAScript module using
  <a href="esm.md#resolver-algorithm-specification">DETECT_MODULE_SYNTAX defined in
  the ESM resolver</a>,
  a. Load X as an ECMAScript module. STOP.
3. THROW the SyntaxError from attempting to parse X as CommonJS in 1. STOP.

LOAD_AS_FILE(X)
1. If X is a file, load X as its file extension format. STOP
2. If X.js is a file,
    a. Find the closest package scope SCOPE to X.
    b. If no scope was found
      1. MAYBE_DETECT_AND_LOAD(X.js)
    c. If the SCOPE/package.json contains "type" field,
      1. If the "type" field is "module", load X.js as an ECMAScript module. STOP.
      2. If the "type" field is "commonjs", load X.js as an CommonJS module. STOP.
    d. MAYBE_DETECT_AND_LOAD(X.js)
3. If X.json is a file, load X.json to a JavaScript Object. STOP
4. If X.node is a file, load X.node as binary addon. STOP

LOAD_INDEX(X)
1. If X/index.js is a file
    a. Find the closest package scope SCOPE to X.
    b. If no scope was found, load X/index.js as a CommonJS module. STOP.
    c. If the SCOPE/package.json contains "type" field,
      1. If the "type" field is "module", load X/index.js as an ECMAScript module. STOP.
      2. Else, load X/index.js as an CommonJS module. STOP.
2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
3. If X/index.node is a file, load X/index.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. If "main" is a falsy value, GOTO 2.
   c. let M = X + (json main field)
   d. LOAD_AS_FILE(M)
   e. LOAD_INDEX(M)
   f. LOAD_INDEX(X) DEPRECATED
   g. THROW "not found"
2. LOAD_INDEX(X)

LOAD_NODE_MODULES(X, START)
1. let DIRS = NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
   a. LOAD_PACKAGE_EXPORTS(X, DIR)
   b. LOAD_AS_FILE(DIR/X)
   c. 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", GOTO d.
   b. DIR = path join(PARTS[0 .. I] + "node_modules")
   c. DIRS = DIR + DIRS
   d. let I = I - 1
5. return DIRS + GLOBAL_FOLDERS

LOAD_PACKAGE_IMPORTS(X, DIR)
1. Find the closest package scope SCOPE to DIR.
2. If no scope was found, return.
3. If the SCOPE/package.json "imports" is null or undefined, return.
4. If `--experimental-require-module` is enabled
  a. let CONDITIONS = ["node", "require", "module-sync"]
  b. Else, let CONDITIONS = ["node", "require"]
5. let MATCH = PACKAGE_IMPORTS_RESOLVE(X, pathToFileURL(SCOPE),
  CONDITIONS) <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
6. RESOLVE_ESM_MATCH(MATCH).

LOAD_PACKAGE_EXPORTS(X, DIR)
1. Try to interpret X as a combination of NAME and SUBPATH where the name
   may have a @scope/ prefix and the subpath begins with a slash (`/`).
2. If X does not match this pattern or DIR/NAME/package.json is not a file,
   return.
3. Parse DIR/NAME/package.json, and look for "exports" field.
4. If "exports" is null or undefined, return.
5. If `--experimental-require-module` is enabled
  a. let CONDITIONS = ["node", "require", "module-sync"]
  b. Else, let CONDITIONS = ["node", "require"]
6. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(DIR/NAME), "." + SUBPATH,
   `package.json` "exports", CONDITIONS) <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
7. RESOLVE_ESM_MATCH(MATCH)

LOAD_PACKAGE_SELF(X, DIR)
1. Find the closest package scope SCOPE to DIR.
2. If no scope was found, return.
3. If the SCOPE/package.json "exports" is null or undefined, return.
4. If the SCOPE/package.json "name" is not the first segment of X, return.
5. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(SCOPE),
   "." + X.slice("name".length), `package.json` "exports", ["node", "require"])
   <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
6. RESOLVE_ESM_MATCH(MATCH)

RESOLVE_ESM_MATCH(MATCH)
1. let RESOLVED_PATH = fileURLToPath(MATCH)
2. If the file at RESOLVED_PATH exists, load RESOLVED_PATH as its extension
   format. STOP
3. THROW "not found" 拷贝