Book 1 · Lesson 1.4

The WASM boundary

Explain why WASM runs sandboxed linear memory
Describe copy-in / compute / copy-out at the JS boundary
Connect to radar 1:1 PII redaction

Prerequisites: 1.3

WebAssembly (WASM) is a portable binary format. Browsers load .wasm modules and call exported functions from JavaScript — with a strict memory boundary.

Why WASM for privacy?

In the radar 1:1 portal, manager notes stay in the browser. Rust code compiled to WASM runs redact_pii_deterministic locally — raw email addresses never POST to your API.

Package: packages/redact-wasm/ · UI: apps/radar/ route /1on1

JavaScript heap

Plaintext lives in the browser only.

copy bytes · run Rust · return string

WASM linear memory

— awaiting call —redact_pii_deterministic()

Boundary rules

Side	Memory	Trust
JavaScript	GC heap	Full browser APIs
WASM	Linear memory	Sandboxed — no DOM, no network unless imported

Crossing the boundary copies strings/bytes — not shared pointers.

Build path (monorepo)

pnpm build:wasm   # wasm-pack → packages/redact-wasm/pkg/

Teach-back prompt

Why is browser WASM redaction different from sending text to the LLM worker?

Load WASM redaction

Run pnpm build:wasm from repo root.
Open radar /1on1 and paste text with an email.
Confirm redaction in browser — check Network tab for no POST of raw email.

Lesson 1.4 check

1. WASM linear memory is… Shared with JS GC Sandboxed from DOM/network by default Same as Postgres Only on server