Troubleshooting issue with v8 callback failure when interacting with JavaScript objects stored on the heap

Question

Troubleshooting issue with v8 callback failure when interacting with JavaScript objects stored on the heap

Creating an instance of a JavaScript object is easy, for example;

myObj1 = new myObject("Object1");

Next step is to call a node.js c++ addon which triggers a callback to the Javascript object...

myAddon = require('myAddon');
myAddon.greet(myObj1.name);

Here is the c++ callback code snippet:

Local<Function> callback = Local<Function>::Cast(args[0]);
Isolate* isolate = args.GetIsolate();
const int argc = 1;
const char *parm = "Hello";
v8::Local<v8::Value> argv[argc] = {  v8::String::NewFromUtf8(isolate, parm) };
callback->Call(isolate->GetCurrentContext()->Global(), argc, argv);

Encountering a problem where the binding doesn't correctly callback to the specified instance of myObject, but instead seems to go back to the base JavaScript class. Thus, causing a loss of instance data.

After extensive research in the past couple of days, it appears that the first parameter passed to the Call() method serves as the "this" pointer in v8. Could the issue lie in using a Global context?

If anyone has insights on how to accurately callback to a JavaScript object stored on the heap, please share.

javascript c++callback node.js-addon

Answer 1

Answer №1

When you call a method like a.b.c(d), the function stored at a.b.c is invoked with the value of a.b being assigned to this within that function's context.

This specific behavior only occurs in this scenario.

For instance, when you execute myAddon.greet(myObj1.name), the greet function from the object myAddon is called with this set to myAddon. No other magic happens here. The detail that the function came from myObj1 is not preserved, making it impossible for your C++ code to reference this object!

In JavaScript, the recommended practice is for the callback-passing code to bind this as needed using the bind() method, such as:

myAddon.greet(myObj1.name.bind(myObj1));

Your C++ code will receive a proxy function object since bind() creates a new function. This proxy will invoke myObj1.name with this set to myObj1, relieving your C++ code from handling it.

To demonstrate, the expression myObj1.name.bind(myObj1) is similar to

(function (o, f) {
    return function () { return f.apply(o, arguments); };
}(myObj1, myObj1.name))

Answer 2

When you call a method like a.b.c(d), the function stored at a.b.c is invoked with the value of a.b being assigned to this within that function's context.

This specific behavior only occurs in this scenario.

For instance, when you execute myAddon.greet(myObj1.name), the greet function from the object myAddon is called with this set to myAddon. No other magic happens here. The detail that the function came from myObj1 is not preserved, making it impossible for your C++ code to reference this object!

In JavaScript, the recommended practice is for the callback-passing code to bind this as needed using the bind() method, such as:

myAddon.greet(myObj1.name.bind(myObj1));

Your C++ code will receive a proxy function object since bind() creates a new function. This proxy will invoke myObj1.name with this set to myObj1, relieving your C++ code from handling it.

To demonstrate, the expression myObj1.name.bind(myObj1) is similar to

(function (o, f) {
    return function () { return f.apply(o, arguments); };
}(myObj1, myObj1.name))

Troubleshooting issue with v8 callback failure when interacting with JavaScript objects stored on the heap

Answer №1

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