Build Your Own Git
Target Audience: Students and Junior Engineers
Language: JavaScript (with Java & Go alternatives)
Duration: 2-3 weeks
Difficulty: ⭐⭐⭐☆☆
Why Build Git?
Git is the most ubiquitous tool in software development, yet most developers treat it as magic. By building your own Git, you’ll:
- Understand content-addressable storage — the elegant idea behind Git’s object model
- Master hashing and cryptography basics — SHA-1 in practice
- Learn tree data structures — directories as trees, commits as a DAG
- Build a real CLI tool — practical software engineering
This is NOT a tutorial on using Git. This is about understanding Git’s internals deeply enough to reimplement them.
Git’s Beautiful Architecture
┌─────────────────────────────────────────────────────────────────────────────┐
│ GIT OBJECT MODEL │
├─────────────────────────────────────────────────────────────────────────────┤
│ │
│ BLOB TREE COMMIT │
│ ───── ───── ────── │
│ File contents Directory listing Snapshot + metadata │
│ SHA-1 of content Points to blobs/trees Points to tree + parent │
│ │
│ ┌───────────┐ ┌───────────────────┐ ┌───────────────────┐ │
│ │ Hello │ │ 100644 hello.txt │ │ tree abc123 │ │
│ │ World │ │ 040000 src/ │ │ parent def456 │ │
│ └───────────┘ └───────────────────┘ │ author John │ │
│ │ │ │ message: Initial │ │
│ └────────────────────────┴───────────────────┘ │
│ │
│ ALL OBJECTS ARE CONTENT-ADDRESSED: │
│ SHA-1(type + size + content) → 40-character hex string │
│ │
└─────────────────────────────────────────────────────────────────────────────┘
What You’ll Build
Core Commands
| Command | Description | Concepts Learned |
|---|
init | Initialize repository | File structure, .git directory |
hash-object | Hash and store a file | SHA-1, zlib compression |
cat-file | Read object contents | Object types, decompression |
add | Stage files | Index file format |
commit | Create commit object | Tree building, commit linking |
log | Show commit history | DAG traversal |
status | Show working tree status | Diff against index |
branch | Manage branches | Refs, symbolic refs |
checkout | Switch branches | Updating HEAD, working tree |
Implementation: JavaScript
Project Structure
mygit/
├── src/
│ ├── commands/
│ │ ├── init.js
│ │ ├── hashObject.js
│ │ ├── catFile.js
│ │ ├── add.js
│ │ ├── commit.js
│ │ ├── log.js
│ │ ├── status.js
│ │ ├── branch.js
│ │ └── checkout.js
│ ├── objects/
│ │ ├── blob.js
│ │ ├── tree.js
│ │ └── commit.js
│ ├── utils/
│ │ ├── hash.js
│ │ ├── compression.js
│ │ ├── index.js
│ │ └── refs.js
│ └── mygit.js
├── package.json
└── README.md
Core Implementation
const crypto = require('crypto');
const zlib = require('zlib');
const fs = require('fs');
const path = require('path');
/**
* Compute SHA-1 hash of content with Git's format
* Git format: "{type} {size}\0{content}"
*/
function hashObject(content, type = 'blob') {
const header = `${type} ${content.length}\0`;
const store = Buffer.concat([Buffer.from(header), Buffer.from(content)]);
const hash = crypto.createHash('sha1').update(store).digest('hex');
return { hash, store };
}
/**
* Write object to .git/objects/{hash[0:2]}/{hash[2:40]}
*/
function writeObject(gitDir, hash, store) {
const objectDir = path.join(gitDir, 'objects', hash.slice(0, 2));
const objectPath = path.join(objectDir, hash.slice(2));
if (!fs.existsSync(objectDir)) {
fs.mkdirSync(objectDir, { recursive: true });
}
// Git stores objects compressed with zlib
const compressed = zlib.deflateSync(store);
fs.writeFileSync(objectPath, compressed);
return hash;
}
/**
* Read object from .git/objects
*/
function readObject(gitDir, hash) {
const objectPath = path.join(
gitDir, 'objects',
hash.slice(0, 2),
hash.slice(2)
);
if (!fs.existsSync(objectPath)) {
throw new Error(`Object ${hash} not found`);
}
const compressed = fs.readFileSync(objectPath);
const store = zlib.inflateSync(compressed);
// Parse header: "{type} {size}\0{content}"
const nullIndex = store.indexOf(0);
const header = store.slice(0, nullIndex).toString();
const [type, size] = header.split(' ');
const content = store.slice(nullIndex + 1);
return { type, size: parseInt(size), content };
}
module.exports = { hashObject, writeObject, readObject };
Exercises
Level 1: Basic Understanding
- Initialize a repository and create a blob manually
- Understand how SHA-1 hashing creates content-addressable storage
- Create a commit and inspect its structure with
cat-file
Level 2: Core Implementation
- Implement the
status command (compare index to working tree)
- Add support for
.gitignore patterns
- Implement
diff to show changes between commits
Level 3: Advanced Features
- Implement merge (fast-forward and three-way)
- Add remote repository support (fetch, push)
- Implement pack files for efficient storage
What You’ve Learned
Content-addressable storage using SHA-1 hashing
Tree data structures for representing directories
DAG (Directed Acyclic Graph) for commit history
Binary file formats (index file)
CLI tool development in JavaScript
Next Steps
