THE INNER WORKINGS OF JAVASCRIPT SOURCE MAPS

Summary from the TLDR Dev newsletter

JavaScript source maps translate minified production code locations back to original source files, letting developers debug with proper variable names and formatting. This happens through VLQ (Variable Length Quantity) encoding with Base64 characters in the `mappings` field, which compresses position data by storing small relative differences between locations rather than absolute coordinates, separated by commas (for columns) and semicolons (for line breaks). Each mapping segment contains 1, 4, or 5 values that specify the generated file column, source file index, source line/column, and optionally the original identifier name, allowing tools to efficiently bridge production code and developer-friendly sources.