Update documentation and error messages

This allows users to get an actionable error message about this
particular problem. We also add detection for this problem.

Signed-off-by: Joe Richey <joerichey@google.com>

src/error.rs

@@ -53,6 +53,9 @@ impl Error {
     pub const NODE_CRYPTO: Error = internal_error(12);
     /// Calling Node.js function `crypto.randomFillSync` failed.
     pub const NODE_RANDOM_FILL_SYNC: Error = internal_error(13);
+    /// Called from an ES module on Node.js. This is unsupported, see:
+    /// https://docs.rs/getrandom#nodejs-es6-module-support.
+    pub const NODE_ES_MODULE: Error = internal_error(14);
 
     /// Codes below this point represent OS Errors (i.e. positive i32 values).
     /// Codes at or above this point, but below [`Error::CUSTOM_START`] are
@@ -170,6 +173,7 @@ fn internal_desc(error: Error) -> Option<&'static str> {
         Error::VXWORKS_RAND_SECURE => Some("randSecure: VxWorks RNG module is not initialized"),
         Error::NODE_CRYPTO => Some("Node.js crypto CommonJS module is unavailable"),
         Error::NODE_RANDOM_FILL_SYNC => Some("Calling Node.js API crypto.randomFillSync failed"),
+        Error::NODE_ES_MODULE => Some("Node.js ES modules are not directly supported, see https://docs.rs/getrandom#nodejs-es6-module-support"),
         _ => None,
     }
 }

src/js.rs

@@ -10,7 +10,7 @@ use crate::Error;
 extern crate std;
 use std::thread_local;
 
-use js_sys::{global, Uint8Array};
+use js_sys::{global, Function, Uint8Array};
 use wasm_bindgen::{prelude::wasm_bindgen, JsCast, JsValue};
 
 // Size of our temporary Uint8Array buffer used with WebCrypto methods
@@ -68,10 +68,14 @@ fn getrandom_init() -> Result<RngSource, Error> {
         c if c.is_object() => c,
         // Node.js CommonJS Crypto module
         _ if is_node(&global) => {
-            match MODULE.require("crypto") {
-                Ok(n) if n.is_object() => return Ok(RngSource::Node(n)),
-                _ => return Err(Error::NODE_CRYPTO),
-            };
+            // If require isn't a valid function, we are in an ES module.
+            match Module::require_fn().dyn_into::<Function>() {
+                Ok(require_fn) => match require_fn.call1(&global, &JsValue::from_str("crypto")) {
+                    Ok(n) => return Ok(RngSource::Node(n.unchecked_into())),
+                    Err(_) => return Err(Error::NODE_CRYPTO),
+                },
+                Err(_) => return Err(Error::NODE_ES_MODULE),
+            }
         }
         // IE 11 Workaround
         _ => match global.ms_crypto() {
@@ -122,10 +126,8 @@ extern "C" {
     // js_name = "module.require", so that Webpack doesn't give a warning. See:
     //   https://github.com/rust-random/getrandom/issues/224
     type Module;
-    #[wasm_bindgen(js_name = module)]
-    static MODULE: Module;
-    #[wasm_bindgen(method, catch)]
-    fn require(this: &Module, s: &str) -> Result<NodeCrypto, JsValue>;
+    #[wasm_bindgen(getter, static_method_of = Module, js_class = module, js_name = require)]
+    fn require_fn() -> JsValue;
 
     // Node JS process Object (https://nodejs.org/api/process.html)
     #[wasm_bindgen(method, getter)]

src/lib.rs

@@ -31,7 +31,7 @@
 //! | Emscripten        | `*‑emscripten`     | `/dev/random` (identical to `/dev/urandom`)
 //! | WASI              | `wasm32‑wasi`      | [`random_get`]
 //! | Web Browser       | `wasm32‑*‑unknown` | [`Crypto.getRandomValues`], see [WebAssembly support]
-//! | Node.js           | `wasm32‑*‑unknown` | [`crypto.randomBytes`], see [WebAssembly support]
+//! | Node.js           | `wasm32‑*‑unknown` | [`crypto.randomFillSync`], see [WebAssembly support]
 //! | SOLID             | `*-kmc-solid_*`    | `SOLID_RNG_SampleRandomBytes`
 //! | Nintendo 3DS      | `armv6k-nintendo-3ds` | [`getrandom`][1]
 //!
@@ -91,6 +91,18 @@
 //!
 //! This feature has no effect on targets other than `wasm32-unknown-unknown`.
 //!
+//! #### Node.js ES module support
+//!
+//! Node.js supports both [CommonJS modules] and [ES modules]. Due to
+//! limitations in wasm-bindgen's [`module`] support, we cannot directly
+//! support ES Modules running on Node.js. However, on Node v15 and later, the
+//! module author can add a simple shim to support the Web Cryptography API:
+//! ```js
+//! import { webcrypto } from 'node:crypto'
+//! globalThis.crypto = webcrypto
+//! ```
+//! This crate will then use the provided `webcrypto` implementation.
+//!
 //! ### Custom implementations
 //!
 //! The [`register_custom_getrandom!`] macro allows a user to mark their own
@@ -154,11 +166,14 @@
 //! [`RDRAND`]: https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide
 //! [`SecRandomCopyBytes`]: https://developer.apple.com/documentation/security/1399291-secrandomcopybytes?language=objc
 //! [`cprng_draw`]: https://fuchsia.dev/fuchsia-src/zircon/syscalls/cprng_draw
-//! [`crypto.randomBytes`]: https://nodejs.org/api/crypto.html#crypto_crypto_randombytes_size_callback
+//! [`crypto.randomFillSync`]: https://nodejs.org/api/crypto.html#cryptorandomfillsyncbuffer-offset-size
 //! [`esp_fill_random`]: https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/random.html#_CPPv415esp_fill_randomPv6size_t
 //! [`random_get`]: https://github.com/WebAssembly/WASI/blob/main/phases/snapshot/docs.md#-random_getbuf-pointeru8-buf_len-size---errno
 //! [WebAssembly support]: #webassembly-support
 //! [`wasm-bindgen`]: https://github.com/rustwasm/wasm-bindgen
+//! [`module`]: https://rustwasm.github.io/wasm-bindgen/reference/attributes/on-js-imports/module.html
+//! [CommonJS modules]: https://nodejs.org/api/modules.html
+//! [ES modules]: https://nodejs.org/api/esm.html
 
 #![doc(
     html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",