Harper

Package

Offline English grammar checker in Rust that runs in the browser via WebAssembly

Price: Free, open source
Access: None, runs locally

About

Harper is Automattic's open-source English grammar and spelling checker. harper.js runs it in the browser or Node.js through WebAssembly and harper-ls brings it to editors, so text never leaves the device. English only; the harper.js API is still marked early access.

What you can do with it

Check grammar and spelling in a web app's text box without sending text to a server
Lint Markdown docs and code comments for English mistakes in VS Code or Neovim
Add on-device English proofreading to an Electron or Node.js app

Get started

Run npm install harper.js
Create a LocalLinter with the bundled binary and call lint()

Example

import { LocalLinter, Dialect } from "harper.js";
import { binary } from "harper.js/binary";

const linter = new LocalLinter({ binary, dialect: Dialect.American });
const lints = await linter.lint("This is a example of how to use harper.js.");
for (const lint of lints) console.log(lint.message(), lint.span());

Details

Hosting: Runs locally
Available in: Worldwide
Official SDKs: JavaScript/TypeScript, Rust
MCP server: None
Works with: VS Code