CONCEPT Cited by 1 source

WebAssembly Exception Handling

Definition

WebAssembly Exception Handling is a Wasm specification proposal adding structured exception primitives — try, catch, catch_all, throw, rethrow — to the Wasm instruction set. Without it, Wasm had no portable way to unwind; the only failure primitive was unreachable, which traps and exits to the embedder.

The proposal reached wide engine support in 2023 and is the primitive underneath Rust's panic=unwind on wasm32-unknown-unknown and C++ exceptions compiled to Wasm.

Two variants

During the proposal's development a late-stage specification change produced two wire-compatible variants:

• Legacy exception handling — the initial variant (proposals/exception-handling/legacy/Exceptions.md). Widely supported; now deprecated.
• Modern exception handling "with exnref" — the final variant (proposals/exception-handling/Exceptions.md) using a reference type for caught exceptions.

As of the 2026-04-22 Cloudflare post, Rust's Wasm targets still default to the legacy variant; Cloudflare is working to make modern the default "next year."

Engine support for modern EH

Runtime	Version	Released
V8 (systems/v8-javascript-engine)	13.8.1	2025-04-28
workerd (systems/workerd)	v1.20250620.0	2025-06-19
Chrome	138	2025-06-28
Firefox	131	2024-10-01
Safari	18.4	2025-03-31
Node.js (systems/nodejs)	25.0.0	2025-10-15

Node.js backport as enabling step

Node.js 24 LTS was the constraint — its release schedule would have kept the ecosystem on legacy EH until April 2028. Cloudflare backported modern EH to Node.js 24 and Node.js 22 so modern EH becomes viable as a default target. "This should allow the modern Exception Handling proposal to become the default target next year." Canonical wiki instance of patterns/upstream-the-fix at the Wasm-engine layer.

What wasm-bindgen uses EH for

Cloudflare's 2026-04 work in systems/wasm-bindgen leverages Wasm EH to:

1. Compile Rust destructors into catch_all blocks that fire on unwind.
2. Compile std::panic::catch_unwind(|| …) into nested try / catch pairs.
3. Surface panics at the Rust↔JS boundary as JavaScript PanicError exceptions.
4. Introduce an Exception.Tag for foreign exceptions to distinguish recoverable unwinds from non-unwindable aborts (see concepts/abort-recovery).

"Since our foreign exception handling was directly using raw WAT-level (WebAssembly text format) Exception Handling instructions already, we found it easier to implement exception tags for foreign exceptions to distinguish them from aborting non-unwind-safe exceptions."

Seen in

• sources/2026-04-22-cloudflare-making-rust-workers-reliable-panic-and-abort-recovery-in-wasm-bindgen — canonical wiki instance. Wasm EH as the primitive Cloudflare builds Rust panic + abort recovery on top of, with concrete engine-support matrix and Node.js backport details.

Related

• concepts/panic-unwind — the Rust-level primitive Wasm EH enables on wasm32-unknown-unknown.
• concepts/panic-abort — the Rust-on-Wasm default before EH was usable.
• concepts/stack-unwinding — the general runtime operation Wasm EH makes possible on this substrate.
• concepts/abort-recovery — uses Wasm EH's Exception.Tag to distinguish recoverable from non-recoverable failures at the JS boundary.
• systems/webassembly — the substrate.
• systems/wasm-bindgen — the Rust-side consumer that turned this primitive into production panic recovery.
• systems/v8-javascript-engine / systems/workerd / systems/nodejs — the engines shipping modern EH.