const fs = require('fs')

// add.js
exports.add = function(a, b) {
    return a + b
}

const { add } = require('./add.js')
add(1, 2) // print 3

// Loads a module at the given file path. Returns that module's
// `exports` property.
Module.prototype.require = function(id) {
    validateString(id, 'id');
    if (id === '') {
        throw new ERR_INVALID_ARG_VALUE('id', id,
                                        'must be a non-empty string');
    }
    requireDepth++;
    try {
        return Module._load(id, this, /* isMain */ false);
    } finally {
        requireDepth--;
    }
};

Module._cache = Object.create(null);
// 這里先定義了一個緩存的對象
// ... ...
// Check the cache for the requested file.
// 1. If a module already exists in the cache: return its exports object.
// 2. If the module is native: call
//    `NativeModule.prototype.compileForPublicLoader()` and return the exports.
// 3. Otherwise, create a new module for the file and save it to the cache.
//    Then have it load  the file contents before returning its exports
//    object.
Module._load = function(request, parent, isMain) {
    let relResolveCacheIdentifier;
    if (parent) {
        debug('Module._load REQUEST %s parent: %s', request, parent.id);
        // Fast path for (lazy loaded) modules in the same directory. The indirect
        // caching is required to allow cache invalidation without changing the old
        // cache key names.
        relResolveCacheIdentifier = `${parent.path}\x00${request}`;
        const filename = relativeResolveCache[relResolveCacheIdentifier];
        if (filename !== undefined) {
            const cachedModule = Module._cache[filename];
            if (cachedModule !== undefined) {
                updateChildren(parent, cachedModule, true);
                return cachedModule.exports;
            }
            delete relativeResolveCache[relResolveCacheIdentifier];
        }
    }
    const filename = Module._resolveFilename(request, parent, isMain);
    const cachedModule = Module._cache[filename];
    if (cachedModule !== undefined) {
        updateChildren(parent, cachedModule, true);
        return cachedModule.exports;
    }
    const mod = loadNativeModule(filename, request, experimentalModules);
    if (mod && mod.canBeRequiredByUsers) return mod.exports;
    // Don't call updateChildren(), Module constructor already does.
    const module = new Module(filename, parent);
    if (isMain) {
        process.mainModule = module;
        module.id = '.';
    }
    Module._cache[filename] = module;
    if (parent !== undefined) {
        relativeResolveCache[relResolveCacheIdentifier] = filename;
    }
    let threw = true;
    try {
        module.load(filename);
        threw = false;
    } finally {
        if (threw) {
            delete Module._cache[filename];
            if (parent !== undefined) {
                delete relativeResolveCache[relResolveCacheIdentifier];
            }
        }
    }
    return module.exports;
};

console.log(module.paths)

node module_path.js

Module._nodeModulePaths = function(from) {
    // Guarantee that 'from' is absolute.
    from = path.resolve(from);
    // note: this approach *only* works when the path is guaranteed
    // to be absolute.  Doing a fully-edge-case-correct path.split
    // that works on both Windows and Posix is non-trivial.
    // return root node_modules when path is 'D:\\'.
    // path.resolve will make sure from.length >=3 in Windows.
    if (from.charCodeAt(from.length - 1) === CHAR_BACKWARD_SLASH &&
        from.charCodeAt(from.length - 2) === CHAR_COLON)
        return [from + 'node_modules'];
    const paths = [];
    var p = 0;
    var last = from.length;
    for (var i = from.length - 1; i >= 0; --i) {
        const code = from.charCodeAt(i);
        // The path segment separator check ('\' and '/') was used to get
        // node_modules path for every path segment.
        // Use colon as an extra condition since we can get node_modules
        // path for drive root like 'C:\node_modules' and don't need to
        // parse drive name.
        if (code === CHAR_BACKWARD_SLASH ||
            code === CHAR_FORWARD_SLASH ||
            code === CHAR_COLON) {
            if (p !== nmLen)
                paths.push(from.slice(0, last) + '\\node_modules');
            last = i;
            p = 0;
        } else if (p !== -1) {
            if (nmChars[p] === code) {
                ++p;
            } else {
                p = -1;
            }
        }
    }
    return paths;
};

// require('./internal/constants.js')
require('./internal/constants')

if (!filename) {
    // Try it with each of the extensions
    if (exts === undefined)
        exts = Object.keys(Module._extensions);
    filename = tryExtensions(basePath, exts, isMain);
}

// Given a path, check if the file exists with any of the set extensions
function tryExtensions(p, exts, isMain) {
    for (var i = 0; i < exts.length; i++) {
        const filename = tryFile(p + exts[i], isMain);
        if (filename) {
            return filename;
        }
    }
    return false;
}

function tryFile(requestPath, isMain) {
    const rc = stat(requestPath);
    if (preserveSymlinks && !isMain) {
        return rc === 0 && path.resolve(requestPath);
    }
    return rc === 0 && toRealPath(requestPath);
}
// Check if the file exists and is not a directory
// if using --preserve-symlinks and isMain is false,
// keep symlinks intact, otherwise resolve to the
// absolute realpath.
function tryFile(requestPath, isMain) {
    const rc = stat(requestPath);
    if (preserveSymlinks && !isMain) {
        return rc === 0 && path.resolve(requestPath);
    }
    return rc === 0 && toRealPath(requestPath);
}
// 這個函數(shù)在其他地方還有用到，比較重要
function toRealPath(requestPath) {
    return fs.realpathSync(requestPath, {
        [internalFS.realpathCacheKey]: realpathCache
    });
}

function Module(id = '', parent) {
    this.id = id;
    this.path = path.dirname(id);
    this.exports = {};
    this.parent = parent;
    updateChildren(parent, this, false);
    this.filename = null;
    this.loaded = false;
    this.children = [];
}

// Enable support to add extra extension types.require.extensions = Module._extensions;

console.log(require.extensions)
// { '.js': [Function], '.json': [Function], '.node': [Function] }

const fs = require('fs')
// 注冊解析方法到 require.extensions 對象
require.extensions['.csv'] = function(module, filename) {
    // module 是當前模塊的 Module 實例，filename 是當前文件模塊的路徑
    const content = fs.readFileSync(filename, 'utf8'),
          lines = content.split(/\r\n/)
    const res = lines.map(line => line.split(','))
    // 注意導出是通過給 module.exports 賦值，而不是用 return
    module.exports = res
}
/*
*    demo.csv 的內容為：
*    1,2,3
*    2,3,4
*    5,6,7
*/
const arr = require('./demo.csv')
console.log(arr)
// output
// [ [ '1', '2', '3' ], [ '2', '3', '4' ], [ '5', '6', '7' ] ]

exports.add = (a, b) => a + b

(function(exports, require, module, __filename, __dirname) {
    exports.add = (a, b) => a + b
})

function changeRef(obj) {
    obj = 12
}
const ref = {}
changeRef(ref)
console.log(ref) // {}

// Native extension for .js
Module._extensions['.js'] = function(module, filename) {
    const content = fs.readFileSync(filename, 'utf8');
    module._compile(content, filename);
};

// Run the file contents in the correct scope or sandbox. Expose
// the correct helper variables (require, module, exports) to
// the file.
// Returns exception, if any.Module.prototype._compile = function(content, filename) {
    let moduleURL;
    let redirects;
    if (manifest) {
        moduleURL = pathToFileURL(filename);
        redirects = manifest.getRedirects(moduleURL);
        manifest.assertIntegrity(moduleURL, content);
    }
    const compiledWrapper = wrapSafe(filename, content);
    var inspectorWrapper = null;
    if (getOptionValue('--inspect-brk') && process._eval == null) {
        if (!resolvedArgv) {
            // We enter the repl if we're not given a filename argument.
            if (process.argv[1]) {
                resolvedArgv = Module._resolveFilename(process.argv[1], null, false);
            } else {
                resolvedArgv = 'repl';
            }
        }
        // Set breakpoint on module start
        if (!hasPausedEntry && filename === resolvedArgv) {
            hasPausedEntry = true;
            inspectorWrapper = internalBinding('inspector').callAndPauseOnStart;
        }
    }
    const dirname = path.dirname(filename);
    const require = makeRequireFunction(this, redirects);
    var result;
    const exports = this.exports;
    const thisValue = exports;
    const module = this;
    if (requireDepth === 0) statCache = new Map();
    if (inspectorWrapper) {
        result = inspectorWrapper(compiledWrapper, thisValue, exports,
                                  require, module, filename, dirname);
    } else {
        result = compiledWrapper.call(thisValue, exports, require, module,
                                      filename, dirname);
    }
    if (requireDepth === 0) statCache = null;
    return result;
};

const compiledWrapper = wrapSafe(filename, content);

function wrapSafe(filename, content) {
    if (patched) {
        const wrapper = Module.wrap(content);
        return vm.runInThisContext(wrapper, {
            filename,
            lineOffset: 0,
            displayErrors: true,
            importModuleDynamically: experimentalModules ? async (specifier) => {
                const loader = await asyncESM.loaderPromise;
                return loader.import(specifier, normalizeReferrerURL(filename));
            } : undefined,
        });
    }
    const compiled = compileFunction(
        content,
        filename,
        0,
        0,
        undefined,
        false,
        undefined,
        [],
        [
            'exports',
            'require',
            'module',
            '__filename',
            '__dirname',
        ]
    );
    if (experimentalModules) {
        const { callbackMap } = internalBinding('module_wrap');
        callbackMap.set(compiled.cacheKey, {
            importModuleDynamically: async (specifier) => {
                const loader = await asyncESM.loaderPromise;
                return loader.import(specifier, normalizeReferrerURL(filename));
            }
        });
    }
    return compiled.function;
}
// Module.wrap
// eslint-disable-next-line func-style
let wrap = function(script) {
    return Module.wrapper[0] + script + Module.wrapper[1];
};
const wrapper = [
    '(function (exports, require, module, __filename, __dirname) { ',
    '\n});'
];
Object.defineProperty(Module, 'wrap', {
    get() {
        return wrap;
    },
    set(value) {
        patched = true;
        wrap = value;
    }
});