Plain-English Explanation: webpack-sources

This library provides a set of tools for handling and manipulating source code during the build process. Think of it as a toolkit that wraps pieces of code and lets you ask questions about them: "What's the actual code?", "How big is it?", "Do you have a source map?" (a source map is a file that helps developers debug code by mapping compiled output back to the original source). The main benefit is that it gives webpack and other build tools a consistent, flexible way to work with code fragments without needing to know whether they came from an original file, have source maps attached, or have been transformed in some way.

At its core, the library defines a base Source class that any piece of code can inherit from. Every source knows how to return its code as text, report its size, provide a source map, and update a hash (useful for detecting changes). The library then provides several specialized source types for common scenarios. For example, RawSource handles plain code with no source map, OriginalSource tracks code that came from an original file, and SourceMapSource stores code along with its source map metadata. There are also decorator classes like CachedSource (which saves computation by remembering results) and ConcatSource (which glues multiple pieces together).

Developers working on build tools like webpack use this library when they need to transform code and keep track of the original file information for debugging. For instance, if a bundler concatenates several JavaScript files together, it can use ConcatSource to combine them while preserving source map data so that errors still point back to the right line in the original files. It's particularly helpful when you're doing complex operations like adding prefixes to code, replacing sections, or managing files that have been processed multiple times.

The design trades simplicity for flexibility: rather than assuming all code is the same, the library lets you wrap code in different ways depending on your situation. This means build tool creators can handle edge cases cleanly and avoid redundant work by caching expensive operations when needed.