v0.2.122

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Notices

• Threading support now requires -Clink-arg=--export=__heap_base to be set
  in RUSTFLAGS for nightly toolchains from 2026-05-06 onward, after
  rust-lang/rust#156174
  removed the implicit __heap_base/__data_end exports on wasm*
  targets. Atomics CI, CLI reference tests, and the nodejs-threads,
  raytrace-parallel, and wasm-audio-worklet examples have been
  updated to pass --export=__heap_base explicitly. The flag is
  backward-compatible with older nightlies.

• -Cpanic=unwind on wasm targets now emits modern (exnref) exception
  handling by default after
  rust-lang/rust#156061,
  and requires Node.js 22.22.3+ (for WebAssembly.JSTag). Legacy EH wasm
  can still be produced on current nightlies by adding
  -Cllvm-args=-wasm-use-legacy-eh to RUSTFLAGS; Node.js 20 may be
  supported with legacy exception handling, with a tracking issue in
  #​5151.

Added

• Implemented TryFromJsValue for Vec<T> where T: TryFromJsValue.
  A JS value converts when it is a real Array (per Array.isArray)
  and every element converts via T::try_from_js_value. This composes
  recursively (Vec<Vec<String>>, Vec<Option<T>>) and works for any
  T with a TryFromJsValue impl, including primitives, String,
  JsValue, and JsCast types. Array-likes (objects with length and
  numeric indices) are intentionally rejected to mirror the static ABI
  representation used by js_value_vector_from_abi.

• New extends_js_class and extends_js_namespace attributes on
  exported structs to allow defining the parent js_class name when
  it has been customized by js_name and the parent's own js_namespace
  as well in turn. New validation is added at code generation time that
  will now catch these cases instead of emitting invalid code. Example:

  #[wasm_bindgen(js_name = "Animal", js_namespace = zoo)]
  pub struct AnimalImpl { /* ... */ }
  
  #[wasm_bindgen(
      extends = AnimalImpl,
      extends_js_class = "Animal",
      extends_js_namespace = zoo,
  )]
  pub struct DogImpl { /* ... */ }

  #​5154

Changed

• When an exported struct uses js_namespace, the corresponding value
  must now be repeated on every impl block. Previously the impl-side
  defaults silently worked resulting in inconsistent emission. Example:

  // Before:
  #[wasm_bindgen(js_namespace = "default")]
  pub struct Counter { /* ... */ }
  
  #[wasm_bindgen]              // worked, but fragile
  impl Counter { /* ... */ }
  
  // After:
  #[wasm_bindgen(js_namespace = "default")]
  pub struct Counter { /* ... */ }
  
  #[wasm_bindgen(js_namespace = "default")]   // now required
  impl Counter { /* ... */ }

  To ease this transition for js_namespace usage, diagnostic
  messages now include hints for missing namespaces for easier
  fixing.

  #​5154

Fixed

• Fixed the descriptor interpreter panicking on Br and BrIf
  instructions emitted by recent nightly compilers when building with
  panic=unwind.
  #​5158

• Emscripten output now works against vanilla upstream emscripten without
  requiring a fork. Dependency tracking, HEAP_DATA_VIEW setup,
  function-decl intrinsic inlining, catch-wrapper gating, and imported
  global handling have all been corrected; ESM imports
  (#[wasm_bindgen(module = "...")] and snippets) are emitted to a
  sidecar library_bindgen.extern-pre.js consumers pass to emcc via
  --extern-pre-js; namespaced exports (js_namespace = [...] on a
  struct/impl) now attach to Module.<segments> instead of emitting
  top-level export const (which emcc's library evaluator rejects);
  the generated .d.ts for namespaced exports is now valid TypeScript
  (mangled identifiers stay module-internal via declare class /
  declare enum / declare function plus export { BindgenModule };
  to mark the file as a module; no spurious unqualified Calc:
  property on BindgenModule for namespaced items; namespace shapes
  land as plain interface members (app: { math: { Calc: typeof app__math__Calc } };) instead of the previously-emitted export let app: { ... }; which was invalid TS1131 syntax inside an
  interface body).
  #​5156

• Fixed a duplicate phantom class being emitted for an exported struct
  renamed via js_name (Rust ident != JS class name) and/or placed in a
  js_namespace, when the struct crosses the boundary as a JsValue
  (e.g. via .into()). The WrapInExportedClass / UnwrapExportedClass
  imports were keyed by the Rust ident rather than the qualified JS name
  that exported_classes is keyed by (a regression from #​5154), so a
  fresh empty class entry was minted and emitted alongside the real one,
  with a free() referencing a nonexistent wasm export. Riding the
  same release's #​5154 wire-format bump, the now-vestigial rust_name
  field is dropped from the schema and the namespace-qualified name is
  no longer cached on AuxStruct, AuxEnum, or ExportedClass
  (derived on demand from (name, js_namespace)), collapsing three
  fallback chains that only papered over the pre-#​5154 keying.

  #​5160

v0.2.121

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Added

• Added the slice_to_array attribute for imported JS functions,
  which makes a &[T] (or Option<&[T]>) argument arrive on the JS
  side as a plain Array rather than a typed array — without
  changing the Rust-side &[T] signature. Useful when binding JS
  APIs that take T[] rather than TypedArray<T>. For primitive
  element kinds the wire is the same zero-copy borrow used by plain
  &[T], with the JS-side shim wrapping the view in Array.from(...)
  to materialise the Array — no extra allocation. For String,
  JsValue, and JS-imported element types the Rust side builds a
  fresh [u32] index buffer that JS reads and frees, with per-element
  &T -> JsValue (refcount bump for handle-shaped types). No T: Clone bound is required. The attribute can be set per-fn
  (#[wasm_bindgen(slice_to_array)] fn ...) or per-block on an
  extern "C" { ... } declaration to apply to every imported function
  in that block. &[ExportedRustStruct] remains unsupported (use
  owned Vec<T> for that). Has no effect on exported functions;
  default &[T] (typed-array view / memory borrow) and owned
  Vec<T> semantics are unchanged for callers that didn't opt in.
  See the
  slice_to_array guide page.
  #​5145

• Added js_sys::AggregateError bindings (constructor, errors getter, and
  new_with_message / new_with_options overloads). AggregateError represents
  multiple unrelated errors wrapped in a single error, e.g. as thrown by
  Promise.any when all input promises reject, along with js_sys::ErrorOptions,
  accepted by built-in error constructors. ErrorOptions::new(cause)
  constructs an instance pre-populated with cause, and get_cause /
  set_cause provide typed access to the property. All standard error
  constructors that previously took only a message (EvalError,
  RangeError, ReferenceError, SyntaxError, TypeError, URIError,
  WebAssembly.CompileError, WebAssembly.LinkError,
  WebAssembly.RuntimeError) now expose a new_with_options(message, &ErrorOptions) overload, and Error gains
  new_with_error_options(message, &ErrorOptions) alongside the existing
  untyped new_with_options. AggregateError::new_with_options also takes
  &ErrorOptions.
  #​5139

• Added inheritance for Rust-exported types: an exported struct may
  declare #[wasm_bindgen(extends = Parent)] to inherit from another
  exported #[wasm_bindgen] struct. The macro injects a hidden
  parent: wasm_bindgen::Parent<Parent> field (a refcounted cell around
  the parent value) and emits class Child extends Parent in the
  generated JS / .d.ts. The child gets an AsRef<Parent<Parent>> impl
  for the direct parent, and threads per-class pointer slots through
  the wasm ABI so that instanceof Parent is true and parent methods
  dispatch soundly via the JS prototype chain. From inside child
  methods, parent data is reached via self.parent.borrow() /
  self.parent.borrow_mut(). See the new
  extends guide page.
  #​5120

• Added js_sys::FinalizationRegistry bindings (constructor, register,
  register_with_token, and unregister). The cleanup callback parameter
  is typed as &Function<fn(JsValue) -> Undefined>, so closures created via
  Closure::new can be passed using Function::from_closure (for owned
  closures retained by JS) or Function::closure_ref (for borrowed scoped
  closures). Pairs with the existing js_sys::WeakRef bindings.
  #​5140

• Added support for well-known symbols in js_name, getter, and
  setter via the explicit bracket-string form
  "[Symbol.<name>]". This works for imported and exported methods,
  fields, getters, and setters. For example,
  #[wasm_bindgen(js_name = "[Symbol.iterator]")] on an exported method
  generates [Symbol.iterator]() { ... } on the generated JS class, and
  the same syntax works for getter / setter and for imported items.
  #​4230

• Added level 2 bindings for ViewTransition to web-sys.
  #​5138

• Add support for dynamic unions: a #[wasm_bindgen] enum that mixes string-literal
  variants with single-field tuple variants is now exported as an untagged TypeScript
  union and dispatched dynamically at the JS↔Rust boundary. The new enum-level
  #[wasm_bindgen(fallback)] attribute makes the last tuple variant an
  unconditional catch-all, supporting unions whose trailing variant has no
  runtime check (e.g., interface-only imports). String enums and dynamic
  unions now emit export type (was bare type) so the alias is a named
  export, and both honour the private flag to suppress the keyword.
  #​4734
  #​2153
  #​2088

Fixed

• From<Promise<T>> for JsFuture<T> and IntoFuture for Promise<T> now
  accept any T: FromWasmAbi (rather than T: JsGeneric), letting
  imported async fns return dynamic-union enums.

• TryFromJsValue for C-style enums no longer accepts non-numeric values
  via JS unary + coercion. Previously calling dyn_into::<MyEnum>() on
  a string would silently coerce it via +"foo" (yielding NaN, then
  NaN as u32 = 0) and could match a discriminant by accident; the
  conversion now returns None for any value that is not a JS number.
  #​4734

• Fix compilation failure with no_std + release
  #​5134

• Raw identifiers (r#name) on enums, enum variants, extern types, statics,
  and impl blocks no longer leak the r# prefix into generated JS / TS
  output and shim names. The Rust-side identifier and the JS-side name are
  now tracked separately for enum variants, and all known identifier
  fallback paths apply Ident::unraw() so e.g.
  pub enum r#Enum { r#A } generates Enum.A instead of producing
  syntactically invalid JS.
  #​4323

• Using the -C panic=unwind option when building for the bundler target
  would produce invalid JS.
  #​5142

Changed

• js_sys::DataView now implements the js_sys::TypedArray trait. A
  FIXME notes that the trait should be renamed to ArrayBufferView in
  the next major release to better reflect the WebIDL spec name covering
  both DataView and the typed-array types.
  #​5135

v0.2.120

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