Self-Hosted Text Diff & Merge Tools: Mergely vs diff2html vs jsdiff

Comparing text files is one of the most fundamental developer workflow tasks. Whether you’re reviewing code changes, comparing configuration files across servers, or auditing document revisions, a good diff tool saves hours of squinting at terminal output. While git diff works in the terminal, a web-based diff tool provides syntax highlighting, intuitive side-by-side navigation, and collaborative sharing capabilities.

In this guide, we compare three open-source self-hosted text diff and merge tools: Mergely (1,272 stars), diff2html (3,380 stars), and jsdiff — the underlying diff algorithm engine that powers many higher-level tools.

Why Self-Host Your Diff Tools?

Pasting sensitive code or configuration files into a public online diff tool like Diffchecker exposes your intellectual property and potentially your infrastructure secrets. API keys, database credentials, and internal architecture details could all leak through a third-party service. A self-hosted diff tool keeps your comparison data on your own server, behind your own authentication.

Beyond security, self-hosting allows deep customization. Add your company’s branding, integrate with internal single sign-on, or extend with custom diff algorithms tailored to your proprietary file formats. For teams doing code review without GitHub or GitLab, a self-hosted diff viewer provides the essential visual review capabilities without depending on external platforms.

For embedded systems, air-gapped environments, and high-security networks, a self-hosted diff tool is often the only viable option. Deploy it on an internal network and compare files without any internet connectivity — essential for defense, finance, and critical infrastructure environments.

For more on self-hosted developer tooling, see our code review platform guide and version-controlled databases comparison.

Feature Comparison

Feature	Mergely	diff2html	jsdiff
Stars	1,272	3,380	N/A (core library)
Language	JavaScript	TypeScript	JavaScript
Type	Full web editor	Diff-to-HTML renderer	Diff algorithm library
Side-by-side view	Yes	Yes	N/A
Inline diff	Yes	Yes	N/A
Merge editing	Yes (interactive)	No	No
Syntax highlighting	Via CodeMirror	Yes (highlight.js)	N/A
File upload	Yes (drag-and-drop)	No (CLI input)	N/A (programmatic)
CLI/API	No	CLI available	Full JavaScript API
Real-time collaboration	No	No	No
Docker support	Manual (nginx wrapper)	Manual	N/A (npm library)
License	AGPL/MIT dual	MIT	BSD
Last Updated	Feb 2026	May 2026	Active (ecosystem)

Mergely: The Full-Featured Diff Editor

Mergely is the most complete solution — a full web-based diff and merge editor, comparable to a simplified version of macOS FileMerge or WinMerge running entirely in your browser. It supports side-by-side comparison, inline diffs, and most uniquely, interactive merge editing where you can selectively apply changes from one side to the other with a single click.

Key Strengths:

Interactive merge editing — click arrow icons to apply changes selectively
Real-time diff updates as you type in either pane
File upload via drag-and-drop or native file picker
LHS/RHS swapping for instant perspective changes
Customizable comparison options (ignore whitespace, case sensitivity)

Self-Hosting Mergely:

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# docker-compose.yml for Mergely
version: "3.8"
services:
  mergely:
    image: nginx:alpine
    container_name: mergely
    volumes:
      - ./mergely-web:/usr/share/nginx/html:ro
    ports:
      - "8090:80"
    restart: unless-stopped

Create the web directory with a self-contained Mergely instance:

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mkdir mergely-web && cd mergely-web

# Download Mergely dependencies
wget https://cdn.jsdelivr.net/npm/mergely@4/lib/mergely.js
wget https://cdn.jsdelivr.net/npm/mergely@4/lib/mergely.css
wget https://code.jquery.com/jquery-3.7.1.min.js
wget https://cdn.jsdelivr.net/npm/codemirror@5/lib/codemirror.js
wget https://cdn.jsdelivr.net/npm/codemirror@5/lib/codemirror.css

Then create your index.html in mergely-web/:

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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Mergely - Self-Hosted Diff Tool</title>
  <script src="jquery-3.7.1.min.js"></script>
  <script src="codemirror.js"></script>
  <link rel="stylesheet" href="codemirror.css">
  <script src="mergely.js"></script>
  <link rel="stylesheet" href="mergely.css">
  <style>
    body, html { margin: 0; padding: 0; height: 100%; }
    #mergely { height: 100vh; }
  </style>
</head>
<body>
  <div id="mergely"></div>
  <script>
    $(document).ready(function() {
      $('#mergely').mergely({
        lhs: function(setValue) { 
          setValue('// Left side - paste original content here'); 
        },
        rhs: function(setValue) { 
          setValue('// Right side - paste modified content here'); 
        },
        cmsettings: {
          lineNumbers: true,
          mode: 'text/plain'
        },
        license: 'lgpl-separate-notice'
      });
    });
  </script>
</body>
</html>

Interactive merge editing allows clicking arrow icons between panes to copy individual changes:

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// Enable merge mode with change application
$('#mergely').mergely({
  lhs: function(setValue) { setValue(originalContent); },
  rhs: function(setValue) { setValue(modifiedContent); },
  cmsettings: {
    lineNumbers: true,
    mode: 'text/x-java'  // Syntax highlighting
  },
  license: 'lgpl-separate-notice',
  // Merge-specific options
  ignorews: true,        // Ignore whitespace changes
  ignorecase: false,     // Case-sensitive comparison
  sidebar: true          // Show change summary sidebar
});

diff2html: The CLI-to-Web Pipeline

diff2html takes a fundamentally different approach — it’s not an interactive editor but a renderer that converts unified diff output into beautiful, syntax-highlighted HTML. This makes it perfect for CI/CD pipelines, automated code review emails, and documentation generation.

Key Strengths:

Consumes standard unified diff format (output of git diff, diff -u)
Produces clean, responsive HTML with syntax highlighting
CLI tool for seamless pipeline integration
Programmatic API for custom integrations
Multiple view modes: side-by-side and line-by-line

Self-Hosting diff2html as a Web Service:

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# docker-compose.yml for diff2html web app
version: "3.8"
services:
  diff2html-api:
    image: node:20-alpine
    container_name: diff2html-api
    working_dir: /app
    command: sh -c "npm init -y && npm install express diff2html-cli && node server.js"
    ports:
      - "8091:3000"
    volumes:
      - ./diffs:/diffs
      - ./output:/output
    restart: unless-stopped

A simple Express.js server that accepts diff input and returns HTML:

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// server.js - Self-hosted diff2html API
const express = require('express');
const { execSync } = require('child_process');
const fs = require('fs');
const path = require('path');
const app = express();

app.use(express.text({ type: 'text/plain', limit: '50mb' }));
app.use(express.static('public'));

app.post('/render', (req, res) => {
  const id = Date.now();
  const inputPath = path.join('/diffs', `${id}.diff`);
  const outputPath = path.join('/output', `${id}.html`);

  fs.writeFileSync(inputPath, req.body);
  execSync(
    `diff2html -i file -s side -F ${outputPath} -- ${inputPath}`,
    { encoding: 'utf-8' }
  );
  
  const html = fs.readFileSync(outputPath, 'utf-8');
  res.send(html);
});

app.get('/health', (req, res) => res.json({ status: 'ok' }));

app.listen(3000, () => console.log('diff2html API on port 3000'));

Using diff2html in automated Git workflows:

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# Generate a review HTML from the last 5 commits
git diff HEAD~5..HEAD > /diffs/recent-changes.diff
diff2html -i file -F /output/review.html -s side -- /diffs/recent-changes.diff

# Generate inline diff with word-level matching
diff2html -i file -F /output/inline-review.html -s line \
  --matching word \
  --highlightCode true \
  -- /diffs/recent-changes.diff

# Multiple files with summary
diff2html -i file -F /output/full-review.html -s side \
  --diffStyle word \
  --showFiles true \
  -- file1.diff file2.diff file3.diff

jsdiff: The Algorithm Engine

jsdiff is the underlying word, line, and character diff algorithm that many higher-level tools (including parts of Mergely) rely on. It’s a pure JavaScript library with no user interface — but understanding it matters because it powers the diff computation behind the scenes and enables custom integrations.

Use cases for jsdiff directly:

Building custom diff UIs with complete design control
Automated systems that need to compute and apply diffs programmatically
Real-time collaborative editing where diffs represent user changes
Server-side diff computation for large files

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const { diffWords, diffLines, diffArrays, diffJson, applyPatch, createPatch } = require('diff');

// Word-level diff (best for prose and documentation)
const wordChanges = diffWords('The quick brown fox', 'The quick red fox');
// Returns: [{value: 'The quick '}, {value: 'brown', removed: true}, {value: 'red', added: true}, ...]

// Line-level diff (best for source code)
const lineChanges = diffLines(oldSourceCode, newSourceCode);

// JSON structural diff (understands JSON semantics)
const oldConfig = { host: 'localhost', port: 3000, debug: true };
const newConfig = { host: '0.0.0.0', port: 8080, debug: true };
const jsonChanges = diffJson(oldConfig, newConfig);

// Create and apply unified diff patches
const patch = createPatch('config.json', 
  JSON.stringify(oldConfig, null, 2),
  JSON.stringify(newConfig, null, 2)
);
const result = applyPatch(JSON.stringify(oldConfig, null, 2), patch);

Choosing the Right Tool

Choose Mergely if: You need an interactive, web-based diff editor with visual merge capabilities. It’s the best drop-in replacement for desktop diff tools and ideal for teams doing manual code review in environments without GitHub or GitLab.

Choose diff2html if: You want to integrate diff visualization into CI/CD pipelines, automated notification systems, or documentation workflows. Its strength is in automation — generate beautiful, consistent diff reports from any git output without manual steps.

Choose jsdiff if: You’re building a custom application that needs diff computation as a core feature — versioned document editing, real-time collaborative text editing, automated merge conflict resolution, or any system where understanding text changes programmatically is essential.

Comparing Output Quality

For a 200-line code change in a Python file:

Metric	Mergely	diff2html	jsdiff (raw)
Visual clarity	High (interactive)	High (styled HTML)	N/A (data only)
Syntax highlighting	Via CodeMirror theme	Via highlight.js	N/A
Change granularity	Line + word	Line + word	Character/word/line
Export formats	None (screen only)	HTML, static site	JSON, patch files
File size overhead	~200KB (editor libs)	~50KB (HTML output)	~3KB (diff data)
Mobile responsive	Yes	Yes	N/A

FAQ

Can Mergely handle very large files?

Mergely loads entire files into browser memory, so files over approximately 50MB may cause performance issues. For very large file comparison, use diff2html on the server side where you can allocate more memory, or use command-line tools like git diff --no-index which can handle files of any size efficiently.

Is there an official Docker image for diff2html?

No official Docker image exists from the diff2html project, but building one is straightforward. Use a Node.js base image, install diff2html-cli globally via npm, and wrap it with a simple Express server or use it directly in CI pipeline jobs. The Docker Compose example above provides a working configuration.

How do these compare to GitHub’s built-in diff viewer?

GitHub’s diff viewer is more feature-rich with commenting, blame annotations, and rich diffs for specific formats like CSV and images. However, it’s tied to the GitHub platform. Mergely provides the closest self-hosted alternative with interactive merge capabilities. diff2html produces visually similar output to GitHub’s side-by-side diff view.

Can I integrate these with pre-commit Git hooks?

Yes — diff2html is excellent for Git hooks. Configure a post-commit hook that generates an HTML diff of the latest commit and saves it to a shared directory or emails it to reviewers. jsdiff is useful in pre-commit hooks for programmatic validation of changes.

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#!/bin/sh
# .git/hooks/post-commit - Generate HTML diff after each commit
diff2html -i file -s side -F /shared/reviews/$(date +%Y%m%d-%H%M%S).html -- \
  <(git diff HEAD~1..HEAD)

Do these support binary or non-text files?

These tools are designed for text comparison only. For binary file comparison like images or PDFs, you need specialized tools. For structured data comparison, JSON-specific diff tools are more appropriate than generic text-based diff tools. For mixed-content repositories, use git diff --stat to identify which files changed, then route text files to diff2html or Mergely.

How do I add authentication to my self-hosted diff tool?

None of these tools include built-in authentication. The recommended approach is placing them behind Nginx with HTTP Basic Auth for simple protection, or using an OAuth2 proxy with your identity provider (Google Workspace, GitHub OAuth, Keycloak) for team use. Configure the reverse proxy to handle all authentication before requests reach the diff application.

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location /diff/ {
    auth_basic "Diff Tools";
    auth_basic_user_file /etc/nginx/.htpasswd;
    proxy_pass http://127.0.0.1:8090/;
}

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