$ show kpdecker/jsdiff
jsdiff is a mature, zero-dependency JavaScript/TypeScript library for computing text differences and manipulating unified diff patches. In production it serves as the diffing engine for code review tools, editors, and version-control tooling: it computes character-, word-, line-, sentence-, CSS-token-, and JSON-level differences between two inputs using Myers' O(ND) algorithm, and can generate, parse, apply, and reverse unified diff patches including Git-style patches with rename/copy/create/delete metadata. The architecture is a clean library design built around an extensible base Diff class (src/diff/base.ts) that implements the core algorithm and delegates tokenization, equality checking, and output joining to subclasses; each diff variant is a thin specialization. The patch subsystem (create/parse/apply/reverse) is cleanly separated from the diff core, and converters emit Google diff-match-patch and XML formats. The public API is exported from src/index.ts with extensive TypeScript overloads supporting synchronous, callback-based asynchronous, and abortable (timeout/maxEditLength) modes. The library ships ESM, CommonJS, and UMD browser bundles and has essentially no runtime dependencies beyond platform APIs like Intl.Segmenter. The project is actively maintained with regular releases and a history of security and performance fixes (ReDoS, O(nĀ³) worst-case behavior). The test suite is extensive, covering Unicode code points, line-ending variations, no-newline-at-EOF markers, malformed patch inputs, and includes dedicated TypeScript type tests (test-d/). Known limitations include formatPatch/parsePatch not being strict inverses for all formatting variations, and Intl.Segmenter-based word tokenization varying across environments. Overall this is a high-craftsmanship library with a large but well-documented API surface.
The LLM identified 5 files worth highlighting for reuse:
Core implementation of Myers' O(ND) difference algorithm. Deviates from the published algorithm by tracking a linked list of change objects per diagonal instead of an array of furthest-reaching D-paths, and skips diagonals that leave the edit graph for linear-time append/truncate cases. The Diff class is explicitly designed for extension - subclasses override tokenize, equals, join, removeEmpty, castInput, and postProcess to create custom diff behaviors. This is the algorithmic heart of the library and could be extracted as a standalone diff engine.
Canonicalizes JSON values by sorting object keys and tracking visited references to handle circular structures, enabling structural diffing of arbitrary JSON objects. The canonicalize function is self-contained and reusable for any JSON normalization or comparison task independent of the diff library.
Robust parser for unified diff and Git-style patches, handling C-style quoted filenames with octal escapes, rename/copy/create/delete metadata, and no-newline-at-EOF markers. Implements a careful state machine over patch text that any tool consuming unified diffs (merge tools, review systems, CI) could reuse.
Applies parsed patches to source text with configurable fuzz factor, automatic line-ending conversion, and hunk position search via a distance iterator. The fuzz-factor matching logic is non-obvious and reusable for any merge or patch-application tool.
A small, dependency-free iterator that scans outward from a target position to find valid hunk fit locations during patch application. Encapsulates a reusable search strategy (checking positions at increasing distance from a guess) applicable to any nearest-match problem.