关于Node.js fs模块stat函数体位置及回调参数来源的技术咨询

Great question—you’re right on the money with your initial guess! Let’s unpack this clearly:

Where do err and stats come from? #

Your callback function isn’t just sitting around waiting—the internal implementation of fs.stat is what calls it and passes those arguments when the file system operation finishes. Here’s the breakdown:

• When you call fs.stat(filepath, yourCallback), Node.js kicks off an asynchronous request to your operating system to retrieve metadata about the file/directory at filepath.
• If something goes wrong during this operation (like the file doesn’t exist, you don’t have permission to access it, or there’s a disk error), Node.js creates a NodeJS.ErrnoException object containing details about the failure, and passes this as the first err argument to your callback.
• If the operation succeeds, Node.js takes the metadata returned by the OS (like file size, creation time, whether it’s a directory) and wraps it into a Stats object, then passes this as the second stats argument to your callback.

In your code example, if filepath points to a non-existent file, the internal fs.stat logic will trigger your callback with a non-null err, which is why your 404 handling code runs.

Where is the stat function body located? #

Node.js core modules (including fs) are a mix of JavaScript and C++ code:

• The JavaScript wrapper for fs.stat lives in the Node.js source code at lib/fs.js. This layer handles argument validation, sets up the asynchronous operation, and manages the callback triggering.
• The actual heavy lifting (talking to the OS file system) is done in C++ code, located in the src/fs_*.cc files in the Node.js repository. This is where the OS-specific file system calls happen.

You don’t need to dive into the C++ code for everyday Node.js development, but if you’re curious, you can explore the Node.js open-source repository to see the full implementation.

内容的提问来源于stack exchange，提问作者Jason Paul