Text Diff Checker

What is a Text Diff Tool?

A text diff (difference) tool compares two versions of text and highlights changes between them. "Diff" originated in Unix systems in the 1970s when developers needed to track changes between file versions. The original `diff` command-line utility, created by Douglas McIlroy and James Hunt at Bell Labs, became essential for software development, version control, and collaborative editing. Today, every version control system (Git, SVN, Mercurial) relies on diff algorithms to show code changes in commits, pull requests, and merges.

Text diff tools answer the fundamental question: "What changed?" Instead of manually scanning two documents word-by-word, the tool automatically identifies additions (text present in the new version but not the old), deletions (text removed from the original), and unchanged content. This visual representation—typically using color coding (green for additions, red for deletions)—makes change tracking fast and accurate.

How Does the Myers Diff Algorithm Work?

Our tool uses the Myers Difference Algorithm, developed by Eugene W. Myers in 1986. The algorithm finds the shortest edit script—the minimum sequence of insertions and deletions needed to transform one text into another. It works by finding the longest common subsequence (LCS) between the two texts. Text that appears in the LCS is unchanged, while everything else represents edits.

The algorithm treats text as a sequence of tokens (in our case, words separated by whitespace). It builds an edit graph where each path from start to finish represents a possible sequence of edits. By finding the shortest path through this graph, the algorithm minimizes the number of highlighted changes, producing clean, readable diffs. The jsdiff library implements this algorithm efficiently in JavaScript, processing comparisons entirely in your browser without sending data to any server.

Common Use Cases

• Version Control: Before committing code changes to Git, developers use diff tools to review exactly what they modified. This prevents accidental commits of debug code or temporary changes.
• Code Review: Pull requests on GitHub, GitLab, and Bitbucket display diffs to show collaborators what changed. Reviewers can comment on specific additions or deletions.
• Document Editing: Writers comparing manuscript versions (original vs. edited by proofreader) can see every word change, deletion, and addition without re-reading entire documents.
• Data Verification: When migrating data between systems, diff tools verify that exports match expectations by comparing before/after snapshots.
• Contract Review: Legal professionals compare contract revisions to ensure only agreed-upon changes were made and nothing was removed without approval.

Diff Output Formats

Traditional diff tools support multiple output formats:

• Unified Format: Shows context lines around changes with `-` for deletions and `+` for additions. Used by Git and patch files.
• Side-by-Side Format: Displays original and modified versions in parallel columns. Good for visual comparison.
• Word-Level Format: Our tool uses this approach, highlighting individual word changes rather than entire lines. This provides more precise change detection.
• Character-Level Format: Highlights individual character changes, useful for finding typos or small edits within long lines.

Frequently Asked Questions (FAQs)