WebAssembly

Horn compiles to WebAssembly, allowing you to validate PDFs directly in the browser with no server required. This powers the Horn web app and can be used in your own applications.

Try the live demo to see it in action.

Installation

Install from npm:

npm install @focusring/horn-wasm

Building from source

Alternatively, build from source. Requires wasm-pack or wasm-bindgen-cli:

# Build the WASM package
cargo build -p horn-wasm --target wasm32-unknown-unknown --release

# Generate JS bindings
wasm-bindgen \
  --target web \
  --out-dir horn-wasm/pkg \
  target/wasm32-unknown-unknown/release/horn_wasm.wasm

# Optional: optimize for size
wasm-opt -O3 --enable-bulk-memory \
  horn-wasm/pkg/horn_wasm_bg.wasm \
  -o horn-wasm/pkg/horn_wasm_bg.wasm

API

The WASM build exposes a single validate function:

/**
 * Validate a PDF from raw bytes.
 *
 * @param name - Display filename (e.g. "report.pdf")
 * @param data - Raw PDF bytes (Uint8Array)
 * @returns A FileReport object with validation results
 */
function validate(name: string, data: Uint8Array): FileReport

Usage in the browser

<script type="module">
  import init, { validate } from './pkg/horn_wasm.js'

  await init()

  const input = document.querySelector('input[type="file"]')
  input.addEventListener('change', async (e) => {
    const file = e.target.files[0]
    const bytes = new Uint8Array(await file.arrayBuffer())
    const report = validate(file.name, bytes)

    console.log(report)
    // {
    //   path: "report.pdf",
    //   standard: "pdf-ua-1",
    //   results: [...],
    //   error: null
    // }
  })
</script>

Usage with a bundler

import init, { validate } from '@focusring/horn-wasm'

await init()

const response = await fetch('/document.pdf')
const bytes = new Uint8Array(await response.arrayBuffer())
const report = validate('document.pdf', bytes)

Return format

The validate function returns the same FileReport structure as the CLI's JSON output:

{
  "path": "document.pdf",
  "standard": "pdf-ua-1",
  "results": [
    {
      "rule_id": "06-001",
      "checkpoint": 6,
      "description": "Document language not set",
      "severity": "error",
      "outcome": {
        "status": "Fail",
        "message": "No /Lang entry in document catalog"
      }
    }
  ],
  "error": null
}

Performance notes

• The WASM build runs single-threaded — parallel processing (rayon) is not available
• Initial PDF structure parsing via pdf_oxide is fast (~4ms per file)
• The lopdf parser is initialized lazily and only triggered when checks need low-level PDF object access
• Complex PDFs that require lopdf may take longer in WASM compared to native (~3s vs ~0.4s) due to single-threaded decompression

Differences from the CLI

Feature	CLI	WASM
Parallel processing	Yes (Rayon)	No (single-threaded)
File system access	Yes	No (bytes only)
Progress bars	Yes	No
Output formatting	text, JSON, SARIF, JUnit	Raw FileReport object
Shell completions	Yes	N/A