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detect whether ES Module is run from command line in Node

Detecting whether an ES module is run from the command line in Node.js can be achieved through various approaches. Let's explore a few common methods:

Using import.meta

The import.meta object provides information about the current module. It includes a url property that contains the URL of the module. You can compare this URL with the current script's file path to determine if the module is being run directly or imported:

HTML ```html
if (import.meta.url === `file://${process.argv[1]}`) {
  // module was not imported but called directly
}
```

Note that you may need to use path.resolve() or path.normalize() to ensure that the file path comparison is accurate across different platforms.

Using module Global Variable

In CommonJS modules, the module global variable is defined. However, in ES modules, this variable is not available. You can leverage this difference to detect the module type:

HTML ```html
const inCommonJs = typeof module !== 'undefined';

console.log(`inCommonJs = ${inCommonJs}`);
```

This approach is straightforward and works in both CommonJS and ES modules.

Using require.main

The require.main property points to the module that was the direct entry point for the Node.js process. If the module is being run as the main script, require.main will be equal to the current module:

HTML ```html
if (require.main === module) {
  // module was executed directly
}
```

This method is particularly useful when dealing with CLI scripts that are executed directly from the command line.

Using pathToFileURL

The pathToFileURL() function can be used to convert a file path to a file URL. This can be helpful for comparing the module URL with the current script's file path:

HTML ```html
import { pathToFileURL } from 'url';

if (import.meta.url === pathToFileURL(process.argv[1]).href) {
  // module was not imported but called directly
}
```

This approach is particularly useful when dealing with Windows paths, where backslashes need to be handled correctly.

Using realpathSync and pathToFileURL

This approach combines the realpathSync() function from the fs module with pathToFileURL() to resolve symlinks and convert the file path to a file URL:

HTML ```html
import { realpathSync } from "fs";
import { pathToFileURL } from "url";

function wasCalledAsScript() {
  // Resolve symlinks
  const realPath = realpathSync(process.argv[1]);

  // Convert file path to file URL
  const realPathAsUrl = pathToFileURL(realPath).href;

  return import.meta.url === realPathAsUrl;
}

if (wasCalledAsScript()) {
  // module was executed and not imported by another file
}
```

This approach is particularly useful for handling CLI scripts that are executed from symlinked paths.

The choice of approach depends on your specific requirements and preferences. Consider factors such as compatibility with different Node.js versions and the specific use case you're targeting.

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