Borrow.js

Borrow.js is a lightweight library that introduces Rust-inspired ownership and borrowing semantics. It enforces safe access patterns at runtime, producing rich, developer-friendly diagnostics when rules are violated.

No transpilers or build plugins are required—everything works out of the box using modern JavaScript Proxies!

Core Concepts

• Ownership: Each managed object has exactly one owner.
• Immutable Borrow: You can create any number of immutable borrows. The original owner cannot mutate the object while these borrows exist.
• Mutable Borrow: You can create exactly one mutable borrow. No other borrows (mutable or immutable) can exist at the same time, and the owner loses access until it's released.
• Move Semantics: Transfer ownership from one reference to another, immediately invalidating the old reference.

Warning

Performance Note: Unlike Rust, where borrowing is a zero-cost abstraction evaluated entirely at compile-time, Borrow.js enforces these rules dynamically at runtime. Due to the heavy use of JavaScript Proxies, structuredClone, and capturing stack traces for rich error diagnostics, this library carries significant performance overhead. I am currently exploring moving to compile-time.

Installation

npm install borrowjs

Usage

1. Ownership & Safe Mutation

import { own } from 'borrowjs';

const user = own({ name: 'Prince', age: 25 });
user.age = 26;

2. Immutable Borrows

import { own, borrow, release } from 'borrowjs';

const user = own({ name: 'Prince' });

// Create an immutable reference
const ref = borrow(user);

console.log(ref.name); // "Prince"

// user.name = "John"; // Throws BorrowError
// ref.name = "John";  // Throws BorrowError

release(ref);

user.name = "John";

3. Scoped Borrows (Automatic Release)

You can use the using keyword to automatically release borrows when they go out of scope, exactly like Rust's lexical lifetimes!

import { own, borrow } from 'borrowjs';

const user = own({ name: 'Prince' });

{
  using ref = borrow(user);
  console.log(ref.name); // "Prince"
  // user.name = "John"; // Throws BorrowError
} // ref is automatically released here!

user.name = "John";

4. Mutable Borrows

import { own, borrowMut, release } from 'borrowjs';

const user = own({ name: 'Prince' });

// Create an exclusive mutable reference
const mutRef = borrowMut(user);

mutRef.name = "Alice";

// user.name = "John"; // Throws BorrowError (Owner locked)
// borrow(user);       // Throws BorrowError (Cannot borrow immutably right now)

release(mutRef);

console.log(user.name); // "Alice"

4. Move Semantics

import { own, move } from 'borrowjs';

const user = own({ name: 'Prince' });
const newUser = move(user);

console.log(newUser.name); // "Prince"
// user.name; // Throws MovedError

Rich Diagnostics

Borrow.js generates highly detailed error messages that pinpoint exactly where conflicts originated, making debugging effortless:

BorrowError: Cannot mutably access property 'name' because the object is currently borrowed immutably.

Active Borrows:
Owner
 │
 ├── Immutable Ref (at myFunction (/src/app.ts:15:10))
 └── Immutable Ref (at /src/app.ts:16:20)

Suggestion: Call release(ref) on active borrows before conflicting operations.

License

MIT