babel-plugin-transform-line

A simple Babel plugin that replaces the identifier __line with the source line number.

Why Use This?

Debugging: Easily track which line of code is executing
Logging: Add line numbers to log messages automatically
Error Reporting: Include source line information in error messages
Zero Runtime Overhead: Line numbers are replaced at build time, not runtime
Minimal Setup: Simple to add to your existing Babel configuration

Installation

npm install --save-dev babel-plugin-transform-line

Usage

Add to your Babel configuration

// In your .babelrc or babel.config.js
{
    "plugins": [
        "babel-plugin-transform-line"
    ]
}

In your code

// Your source code
console.log(`Error occurred on line ${__line}`);

// After transformation
console.log(`Error occurred on line ${12}`); // Where 12 is the actual line number

Examples

Simple Usage with Log Statements

const _processData = data => {
    if (!data) {
        console.error(`[Line ${__line}] Invalid data provided`); // [Line 3] Invalid data provided

        return null;
    }

    // Process data
    console.log(`[Line ${__line}] Processing completed`); // [Line 9] Processing completed

    return data;
};

Enhanced Error Objects

import _Error from 'isotropic-error';

try {
    // Some code that might fail
    throw _Error({
        details: {
            line: __line
        },
        message: 'Encountered an error'
    });
} catch (error) {
    console.error(error.details.line); // 7
}

How It Works

This plugin uses Babel's AST visitor pattern to find every occurrence of the __line identifier in your code. When found, it replaces it with a numeric literal representing the source line number where __line appears.

If line information isn't available (which is rare and usually only happens in programmatically generated ASTs), the plugin replaces __line with void 0 (undefined).

Notes

The plugin only transforms references to the exact identifier __line. Variables with similar names are not affected.
Line numbers refer to the original source code, not the transformed output.
This plugin is most helpful during development and debugging. Consider removing it in production builds if line numbers aren't needed.

Use Cases

Debugging: Quickly identify which line of code is executing
Logging: Add contextual information to log messages
Error Reporting: Create more informative error messages
Testing: Reference line numbers in test output for easier debugging
Documentation: Auto-generate code examples with line numbers

Integration with Other Tools

With Isotropic Logger

import _logger from 'isotropic-logger';

// Usage - use __line at the call site
const _processUser = user => {
    _logger.info({
        line: __line,
        user
    }, 'Processing user');

    if (!user.id) {
        _logger.error({
            line: __line
        }, 'User missing id');

        return false;
    }

  return true;
};

With Mocha and Chai

import _chai from 'isotropic-dev-dependencies/lib/chai.js';
import _mocha from 'isotropic-dev-dependencies/lib/mocha.js';

// Don't actually use this validation function for real
const _validateEmail = email => email !== null && email !== '';

_mocha.describe('Data validation', () => {
    _mocha.it('should validate email addresses', () => {
        const email = 'test@example.com',
            result = _validateEmail(email);

        // Include line number in assertion message
        _chai.expect(result, `Email validation failed on line ${__line}`).to.be.true;
    });

    _mocha.it('should handle invalid inputs properly', () => {
        const badInput = null;

        // Easy to see which test assertion failed
        _chai.expect(_validateEmail(badInput), `Null validation failed on line ${__line}`).to.be.false;
        _chai.expect(_validateEmail(''), `Empty string validation failed on line ${__line}`).to.be.false;
    });
});

Important Usage Notes

Remember that __line is replaced with the literal line number where it appears:

// INCORRECT USAGE - You usually don't want to do this!
const LINE = __line; // This becomes const LINE = 2;

// Later in your code
console.log(`This is line ${LINE}`); // Will always print "This is line 2"

// CORRECT USAGE - Use __line directly where you need the line number
console.log(`This is line ${__line}`); // Will print the current line number

Contributing

Please refer to CONTRIBUTING.md for contribution guidelines.

Issues

If you encounter any issues, please file them at https://github.com/ibi-group/babel-plugin-transform-line/issues