Dave Hulbert's Today I Learned (TIL)

Making JavaScript Behave Like Python for Script and Module Distinctions with Bun

While working on a JavaScript project using the chalk library to stylize terminal output, I found an interesting thing about handling JavaScript files as both importable modules and executable scripts, much like Python's if __name__ == "__main__".

I wanted to be able to quickly test and debug a module like this:

bun run src/util/styles.ts
# shows helpful debug info

bun run src/index.ts
# imports src/util/styles.ts but doesn't show debugging info

The Solution

Here's the code that makes it work:

import chalk from 'chalk';

const styles = {
    error: chalk.bold.underline.red,
    warning: chalk.italic.inverse.hex('#FFA500'),
    info: chalk.blue

// Ensuring the file behaves as intended when compiled with Bun
if (import.meta.main && import.meta.file === 'chalkStyles.ts') {
    // Demonstrating the styles in the console
    for (const [styleName, style] of Object.entries(styles)) {
        console.log(`${style('This is a test!')}\t${styleName}`);

export default styles;


In Python, it's common to use if __name__ == "__main__" to determine whether a script is being run standalone or being imported. I wanted to achieve something similar in JavaScript to allow for both importing styles for use in other parts of an application and running the script directly to test the styles.

The JavaScript Way

In JavaScript, especially when using ES Modules, you can use import.meta.main. This property is true if the script is run directly, which seemed like the perfect counterpart to Python’s approach.

However, when I was compiling my JavaScript code using Bun (a modern JavaScript runtime like Node.js), just using import.meta.main wasn't sufficient due to the way Bun handles module compilation.

To accurately determine the context when compiled, I added an extra check: import.meta.file === 'styles.ts'. This check ensures that not only is the file executed as the main entry point, but it is also specifically the chalkStyles.ts file, avoiding any misinterpretation when multiple files are involved in the compilation.

Implementing this dual check allowed me to flexibly use the styles module while quickly testing it as a standalone script.