How do you implement a custom module loader in Node.js?

Question

Accepted Answer

**Custom module loader** — intercept and transform module loading.

```javascript
// Using Module._extensions (internal API)
const Module = require("module");
const originalResolveFilename = Module._resolveFilename;

Module._resolveFilename = function(request, parent) {
    // Custom alias resolution
    const aliases = {
        "@services": "./src/services",
        "@utils": "./src/utils",
    };

for (const [alias, path] of Object.entries(aliases)) {
        if (request.startsWith(alias)) {
            request = request.replace(alias, path);
        }
    }

return originalResolveFilename.call(this, request, parent);
};

// Custom file extension
Module._extensions[".yaml"] = function(module, filename) {
    const fs = require("fs");
    const yaml = require("js-yaml");
    const content = fs.readFileSync(filename, "utf8");
    module.exports = yaml.load(content);
};

// Now you can require YAML files
const config = require("./config.yaml");
```

**ESM loader (Node 16+):**
```javascript
// loader.mjs
export function resolve(specifier, context, nextResolve) {
    if (specifier.startsWith("@/")) {
        return nextResolve(specifier.replace("@/", "./src/"), context);
    }
    return nextResolve(specifier, context);
}
// node --loader ./loader.mjs app.js
```

Node.js: How do you implement a custom module loader in Node.js?