Executing a JavaScript function from V8 in C++: A Step-by-Step Guide

Question

Executing a JavaScript function from V8 in C++: A Step-by-Step Guide

When I want to run a basic Javascript program using v8, I follow these steps:

// Define a string containing the JavaScript source code.
v8::Local<v8::String> source = v8::String::NewFromUtf8(isolate, "'Hello' + ', from Javascript!'", v8::NewStringType::kNormal).ToLocalChecked();

// Compile the provided source code.
v8::Local<v8::Script> script = v8::Script::Compile(context, source).ToLocalChecked();

// Execute the script to get the result.
v8::Local<v8::Value> result = script->Run(context).ToLocalChecked();

If I want to call a Javascript function in /path/to/my_js_functions.js file, how can I achieve this?

function myJsFunction(stringParam) {
  return stringParam   // The function simply returns the input parameter
}

I appreciate any guidance. Thank you!

javascript c++v8

Answer 1

Answer №1

To start, you need to obtain the function object. Assuming it resides in the global scope (meaning it's on the global object), you can access it like this:

v8::Local<v8::String> name = v8::String::NewFromUtf8(
    isolate, "myJsFunction", v8::NewStringType::kInternalized).ToLocalChecked();
v8::Local<v8::Value> obj =
    context->Global()->Get(context.local(), name).ToLocalChecked();
if (!obj->IsFunction()) {
  /* Handle error if someone overwrote it */
}
v8::Local<v8::Function> my_function = v8::Local<v8::Function>::Cast(obj);

Keep in mind that whenever you receive a MaybeLocal, the result may be empty if an exception was thrown. If there's no guarantee against exceptions, avoid using just .ToLocalChecked() (as it will crash when the MaybeLocal is empty) and properly account for and manage the error scenario.

Once you have the function, you can prepare arguments for it and invoke it:

v8::Local<v8::Value> receiver = ...;
int argc = ...;
v8::Local<v8::Value> args[argc] = ...;
v8::MaybeLocal<v8::Value> result = my_function->Call(context.local(), receiver, argc, args);

Again, the result comes as a MaybeLocal because functions can throw exceptions (either explicitly or indirectly through other calls/actions). It's your responsibility to handle any errors and then appropriately convert the result to its intended type for further processing.

(Interacting with JavaScript via a C++ API can be challenging due to the significant conceptual differences between the two languages.)

For more examples, refer to V8's test-api.cc.

Answer 2

To start, you need to obtain the function object. Assuming it resides in the global scope (meaning it's on the global object), you can access it like this:

v8::Local<v8::String> name = v8::String::NewFromUtf8(
    isolate, "myJsFunction", v8::NewStringType::kInternalized).ToLocalChecked();
v8::Local<v8::Value> obj =
    context->Global()->Get(context.local(), name).ToLocalChecked();
if (!obj->IsFunction()) {
  /* Handle error if someone overwrote it */
}
v8::Local<v8::Function> my_function = v8::Local<v8::Function>::Cast(obj);

Keep in mind that whenever you receive a MaybeLocal, the result may be empty if an exception was thrown. If there's no guarantee against exceptions, avoid using just .ToLocalChecked() (as it will crash when the MaybeLocal is empty) and properly account for and manage the error scenario.

Once you have the function, you can prepare arguments for it and invoke it:

v8::Local<v8::Value> receiver = ...;
int argc = ...;
v8::Local<v8::Value> args[argc] = ...;
v8::MaybeLocal<v8::Value> result = my_function->Call(context.local(), receiver, argc, args);

Again, the result comes as a MaybeLocal because functions can throw exceptions (either explicitly or indirectly through other calls/actions). It's your responsibility to handle any errors and then appropriately convert the result to its intended type for further processing.

(Interacting with JavaScript via a C++ API can be challenging due to the significant conceptual differences between the two languages.)

For more examples, refer to V8's test-api.cc.

Executing a JavaScript function from V8 in C++: A Step-by-Step Guide

Answer №1

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