From 7bdcc609880a183af8a0aecd2c2d402cc66ee802 Mon Sep 17 00:00:00 2001
From: Ted Campbell <tcampbell@mozilla.com>
Date: Tue, 1 Sep 2020 09:22:41 -0400
Subject: [PATCH] examples: Add module embedding example

This closes Issue #22.
---
 examples/README.md   |   1 +
 examples/modules.cpp | 121 +++++++++++++++++++++++++++++++++++++++++++
 meson.build          |   1 +
 3 files changed, 123 insertions(+)
 create mode 100644 examples/modules.cpp

diff --git a/examples/README.md b/examples/README.md
index 025fe1d..fe976c4 100644
--- a/examples/README.md
+++ b/examples/README.md
@@ -56,3 +56,4 @@ in the `meson.build` file, and add a description of it to this
   lazy property resolution.
   Use this in cases where defining properties and methods in your class
   upfront might be slow.
+- **modules.cpp** - Example of how to load ES Module sources.
diff --git a/examples/modules.cpp b/examples/modules.cpp
new file mode 100644
index 0000000..cb6ab15
--- /dev/null
+++ b/examples/modules.cpp
@@ -0,0 +1,121 @@
+#include <map>
+#include <string>
+
+#include <jsapi.h>
+
+#include <js/CompilationAndEvaluation.h>
+#include <js/Modules.h>
+#include <js/SourceText.h>
+
+#include "boilerplate.h"
+
+// This examples demonstrates how to compile ES modules in an embedding.
+//
+// See 'boilerplate.cpp' for the parts of this example that are reused in many
+// simple embedding examples.
+
+// Translates source code into a JSObject representing the compiled module. This
+// module is not yet linked/instantiated.
+static JSObject* CompileExampleModule(JSContext* cx, const char* filename,
+                                      const char* code) {
+  JS::CompileOptions options(cx);
+  options.setFileAndLine(filename, 1);
+
+  JS::SourceText<mozilla::Utf8Unit> source;
+  if (!source.init(cx, code, strlen(code), JS::SourceOwnership::Borrowed)) {
+    return nullptr;
+  }
+
+  // Compile the module source to bytecode.
+  //
+  // NOTE: This generates a JSObject instead of a JSScript. This contains
+  // additional metadata to resolve imports/exports. This object should not be
+  // exposed to other JS code or unexpected behaviour may occur.
+  return JS::CompileModule(cx, options, source);
+}
+
+// Maintain a registry of imported modules. The ResolveHook may be called
+// multiple times for the same specifier and we need to return the same compiled
+// module.
+//
+// NOTE: This example assumes only one JSContext/GlobalObject is used, but in
+// general the registry needs to be distinct for each GlobalObject.
+static std::map<std::u16string, JS::PersistentRootedObject> moduleRegistry;
+
+// Callback for embedding to provide modules for import statements. This example
+// hardcodes sources, but an embedding would normally load files here.
+static JSObject* ExampleResolveHook(JSContext* cx,
+                                    JS::HandleValue modulePrivate,
+                                    JS::HandleString spec) {
+  // Convert specifier to a std::u16char for simplicity.
+  JS::UniqueTwoByteChars specChars(JS_CopyStringCharsZ(cx, spec));
+  if (!specChars) {
+    return nullptr;
+  }
+  std::u16string filename(specChars.get());
+
+  // If we already resolved before, return same module.
+  auto search = moduleRegistry.find(filename);
+  if (search != moduleRegistry.end()) {
+    return search->second;
+  }
+
+  JS::RootedObject mod(cx);
+
+  if (filename == u"a") {
+    mod = CompileExampleModule(cx, "a", "export const C1 = 1;");
+    if (!mod) {
+      return nullptr;
+    }
+  }
+
+  // Register result in table.
+  if (mod) {
+    moduleRegistry.emplace(filename, JS::PersistentRootedObject(cx, mod));
+    return mod;
+  }
+
+  JS_ReportErrorASCII(cx, "Cannot resolve import specifier");
+  return nullptr;
+}
+
+static bool ModuleExample(JSContext* cx) {
+  JS::RootedObject global(cx, boilerplate::CreateGlobal(cx));
+  if (!global) {
+    return false;
+  }
+
+  JSAutoRealm ar(cx, global);
+
+  // Register a hook in order to provide modules
+  JS::SetModuleResolveHook(JS_GetRuntime(cx), ExampleResolveHook);
+
+  // Compile the top module.
+  JS::RootedObject mod(
+      cx, CompileExampleModule(cx, "top", "import {C1} from 'a';"));
+  if (!mod) {
+    boilerplate::ReportAndClearException(cx);
+    return false;
+  }
+
+  // Resolve imports by loading and compiling additional scripts.
+  if (!JS::ModuleInstantiate(cx, mod)) {
+    boilerplate::ReportAndClearException(cx);
+    return false;
+  }
+
+  // Execute the module bytecode.
+  if (!JS::ModuleEvaluate(cx, mod)) {
+    boilerplate::ReportAndClearException(cx);
+    return false;
+  }
+
+  return true;
+}
+
+int main(int argc, const char* argv[]) {
+  if (!boilerplate::RunExample(ModuleExample)) {
+    return 1;
+  }
+  return 0;
+}
diff --git a/meson.build b/meson.build
index 248ffbf..1ac775d 100644
--- a/meson.build
+++ b/meson.build
@@ -70,3 +70,4 @@ executable('cookbook', 'examples/cookbook.cpp', 'examples/boilerplate.cpp', depe
 executable('repl', 'examples/repl.cpp', 'examples/boilerplate.cpp', dependencies: [spidermonkey, readline])
 executable('tracing', 'examples/tracing.cpp', 'examples/boilerplate.cpp', dependencies: spidermonkey)
 executable('resolve', 'examples/resolve.cpp', 'examples/boilerplate.cpp', dependencies: [spidermonkey, zlib])
+executable('modules', 'examples/modules.cpp', 'examples/boilerplate.cpp', dependencies: [spidermonkey])