Text Diff Checker - Compare Two Texts Online

How Diff Works

A diff algorithm compares two texts and produces the shortest sequence of edits — insertions and deletions — needed to transform one into the other. The most widely used algorithm is Myers diff (published 1986), which powers Git, GitHub, and most modern diff tools.

Standard diff output uses a unified format:

• Lines starting with + are additions (in the new version)
• Lines starting with - are deletions (from the old version)
• Lines starting with a space are context (unchanged)

Common Use Cases

• Code review: Compare a function before and after refactoring to verify that logic was not accidentally changed.
• Configuration auditing: Diff config files between environments (dev vs. production) to find unauthorized or unexpected differences.
• API response comparison: Paste the JSON responses from two API versions side by side to spot breaking changes.
• Document proofreading: Identify what changed between two drafts of a document or legal agreement.
• Database query output: Compare query results before and after an index change or schema migration.
• Merge conflict resolution: Understand exactly what two branches changed about the same section of code.

Diff in the Terminal

Unix diff command

# Basic line diff
diff original.txt modified.txt

# Unified format (what Git uses)
diff -u original.txt modified.txt

# Side-by-side comparison
diff -y original.txt modified.txt

# Ignore whitespace
diff -w original.txt modified.txt

Git diff

# Show unstaged changes
git diff

# Compare two branches
git diff main..feature-branch

# Compare specific files between commits
git diff HEAD~1 HEAD -- path/to/file.ts

# Word-level diff (highlight changed words, not lines)
git diff --word-diff

Reading a Diff — Example

@@ -1,5 +1,6 @@
 function greet(name) {
-  return "Hello " + name;
+  return `Hello, ${name}!`;
+  // Updated to use template literals
 }

The @@ line shows the position: lines 1–5 in the original (-1,5), lines 1–6 in the new version (+1,6). The red line was removed, the two green lines were added.

Tips for Effective Diffing

• Normalize whitespace and line endings before diffing to avoid noise from formatting-only changes.
• Use semantic diff tools (e.g., ast-diff for code) when structural changes matter more than textual ones.
• For large files, focus on changed sections — most diff tools support folding unchanged context.
• When comparing JSON or YAML, format/sort keys first so structural differences are not obscured by key ordering.