src: add a method to get string encoding info

doc/api/v8.md

@@ -1304,6 +1304,45 @@ setTimeout(() => {
 }, 1000);
 ```
 
+## `v8.isStringOneByteRepresentation(content)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `content` {string}
+* Returns: {boolean}
+
+V8 only supports `Latin-1/ISO-8859-1` and `UTF16` as the underlying representation of a string.
+If the `content` uses `Latin-1/ISO-8859-1` as the underlying representation, this function will return true;
+otherwise, it returns false.
+
+If this method returns false, that does not mean that the string contains some characters not in `Latin-1/ISO-8859-1`.
+Sometimes a `Latin-1` string may also be represented as `UTF16`.
+
+```js
+const { isStringOneByteRepresentation } = require('node:v8');
+
+const Encoding = {
+  latin1: 1,
+  utf16le: 2,
+};
+const buffer = Buffer.alloc(100);
+function writeString(input) {
+  if (isStringOneByteRepresentation(input)) {
+    buffer.writeUint8(Encoding.latin1);
+    buffer.writeUint32LE(input.length, 1);
+    buffer.write(input, 5, 'latin1');
+  } else {
+    buffer.writeUint8(Encoding.utf16le);
+    buffer.writeUint32LE(input.length * 2, 1);
+    buffer.write(input, 5, 'utf16le');
+  }
+}
+writeString('hello');
+writeString('你好');
+```
+
 [HTML structured clone algorithm]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm
 [Hook Callbacks]: #hook-callbacks
 [V8]: https://developers.google.com/v8/

lib/v8.js

@@ -104,6 +104,7 @@ const binding = internalBinding('v8');
 const {
   cachedDataVersionTag,
   setFlagsFromString: _setFlagsFromString,
+  isStringOneByteRepresentation: _isStringOneByteRepresentation,
   updateHeapStatisticsBuffer,
   updateHeapSpaceStatisticsBuffer,
   updateHeapCodeStatisticsBuffer,
@@ -155,6 +156,17 @@ function setFlagsFromString(flags) {
   _setFlagsFromString(flags);
 }
 
+/**
+ * Return whether this string uses one byte as underlying representation or not.
+ * @param {string} content
+ * @returns {boolean}
+ */
+function isStringOneByteRepresentation(content) {
+  validateString(content, 'content');
+  return _isStringOneByteRepresentation(content);
+}
+
+
 /**
  * Gets the current V8 heap statistics.
  * @returns {{
@@ -439,4 +451,5 @@ module.exports = {
   startupSnapshot,
   setHeapSnapshotNearHeapLimit,
   GCProfiler,
+  isStringOneByteRepresentation,
 };

src/node_external_reference.h

@@ -12,6 +12,9 @@ namespace node {
 
 using CFunctionCallbackWithOneByteString =
     uint32_t (*)(v8::Local<v8::Value>, const v8::FastOneByteString&);
+
+using CFunctionCallbackReturnBool = bool (*)(v8::Local<v8::Value> unused,
+                                             v8::Local<v8::Value> receiver);
 using CFunctionCallback = void (*)(v8::Local<v8::Value> unused,
                                    v8::Local<v8::Value> receiver);
 using CFunctionCallbackReturnDouble =
@@ -90,6 +93,7 @@ class ExternalReferenceRegistry {
 #define ALLOWED_EXTERNAL_REFERENCE_TYPES(V)                                    \
   V(CFunctionCallback)                                                         \
   V(CFunctionCallbackWithOneByteString)                                        \
+  V(CFunctionCallbackReturnBool)                                               \
   V(CFunctionCallbackReturnDouble)                                             \
   V(CFunctionCallbackReturnInt32)                                              \
   V(CFunctionCallbackValueReturnDouble)                                        \

src/node_v8.cc

@@ -32,6 +32,7 @@
 namespace node {
 namespace v8_utils {
 using v8::Array;
+using v8::CFunction;
 using v8::Context;
 using v8::FunctionCallbackInfo;
 using v8::FunctionTemplate;
@@ -238,6 +239,23 @@ void SetFlagsFromString(const FunctionCallbackInfo<Value>& args) {
   V8::SetFlagsFromString(*flags, static_cast<size_t>(flags.length()));
 }
 
+static void IsStringOneByteRepresentation(
+    const FunctionCallbackInfo<Value>& args) {
+  CHECK_EQ(args.Length(), 1);
+  CHECK(args[0]->IsString());
+  bool is_one_byte = args[0].As<String>()->IsOneByte();
+  args.GetReturnValue().Set(is_one_byte);
+}
+
+static bool FastIsStringOneByteRepresentation(Local<Value> receiver,
+                                              const Local<Value> target) {
+  CHECK(target->IsString());
+  return target.As<String>()->IsOneByte();
+}
+
+CFunction fast_is_string_one_byte_representation_(
+    CFunction::Make(FastIsStringOneByteRepresentation));
+
 static const char* GetGCTypeName(v8::GCType gc_type) {
   switch (gc_type) {
     case v8::GCType::kGCTypeScavenge:
@@ -479,6 +497,13 @@ void Initialize(Local<Object> target,
   // Export symbols used by v8.setFlagsFromString()
   SetMethod(context, target, "setFlagsFromString", SetFlagsFromString);
 
+  // Export symbols used by v8.isStringOneByteRepresentation()
+  SetFastMethodNoSideEffect(context,
+                            target,
+                            "isStringOneByteRepresentation",
+                            IsStringOneByteRepresentation,
+                            &fast_is_string_one_byte_representation_);
+
   // GCProfiler
   Local<FunctionTemplate> t =
       NewFunctionTemplate(env->isolate(), GCProfiler::New);
@@ -498,6 +523,9 @@ void RegisterExternalReferences(ExternalReferenceRegistry* registry) {
   registry->Register(GCProfiler::New);
   registry->Register(GCProfiler::Start);
   registry->Register(GCProfiler::Stop);
+  registry->Register(IsStringOneByteRepresentation);
+  registry->Register(FastIsStringOneByteRepresentation);
+  registry->Register(fast_is_string_one_byte_representation_.GetTypeInfo());
 }
 
 }  // namespace v8_utils

test/parallel/test-v8-string-is-one-byte-representation.js

@@ -0,0 +1,36 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const { isStringOneByteRepresentation } = require('v8');
+
+[
+  undefined,
+  null,
+  false,
+  5n,
+  5,
+  Symbol(),
+  () => {},
+  {},
+].forEach((value) => {
+  assert.throws(
+    () => { isStringOneByteRepresentation(value); },
+    /The "content" argument must be of type string/
+  );
+});
+
+{
+  const latin1String = 'hello world!';
+  // Run this inside a for loop to trigger the fast API
+  for (let i = 0; i < 10_000; i++) {
+    assert.strictEqual(isStringOneByteRepresentation(latin1String), true);
+  }
+}
+
+{
+  const utf16String = '你好😀😃';
+  // Run this inside a for loop to trigger the fast API
+  for (let i = 0; i < 10_000; i++) {
+    assert.strictEqual(isStringOneByteRepresentation(utf16String), false);
+  }
+}