Type conversions from JavaScript to C++ in V8

[ingve]

http://blog.scottfrees.com/type-conversions-from-javascript-to-c-in-v8

[megalodon]

Somewhat related: You can also read and manipulate the contents of JavaScript typed arrays in V8:

    // js
    var x = new Float64Array([1, 2, 3]);
    binding(x);

    // c++
    void binding(const v8::FunctionCallbackInfo<v8::Value>& info) {
        void *data = info[0].As<v8::Float64Array>()->Buffer()->GetContents().Data();
        double *x = reinterpret_cast<double*>(data);

        // do whatever, e.g.
        x[0] = 0;
    }

[dkopi → megalodon]

Be ultra careful when doing this. First of all - make sure to get, validate and store the buffer's length (ByteLength())

Secondly - GetContents() is only for temporary access to a buffer. if you store that pointer and use it later (//do whatever), there's a great chance it will get garbage collected. Make sure to Externalize() the array if you intend on keeping the pointer for later.

[zbjornson → megalodon]

Or use the relatively new NaN helper TypedArrayContents:

https://github.com/nodejs/nan/blob/master/doc/v8_misc.md#api...

[wasdfan]

Does anybody know of any good articles that discuss the actual performance merits of using npm addons? Aside from raw computation heavy functions (whereby dropping down to C++ would seem to be a good idea a priori), are there are other scenarios where using npm addons is a good idea in the node context?

For example, one thing I have found is that nodejs seems to be horrendously very slow at file system manipulation. Dropping down to a C++ addon thus might be a good idea if you're making a nodejs app that has a lot of file system manipulation involved.

[dkopi]

Does V8 have easy conversion between V8 classes and std classes?

converting a V8 string to an std string and back by first casting to a null terminated char array seems very cumbersome and error prone.

[tibiapejagala → dkopi]

It would make sense to provide some.

Example problem could be that both JavaScript strings and std::string can have '\0' bytes in them. Converting them like in the post would lead to string truncation if a null byte was present somewhere in the middle of a string.

So it should be something like (error checking aside): v8::String::Utf8Value s(args[0]); std::string str(*s, s.length());

[wasdfan]

This is eerily timely (recently have been dealing with npm addons). Glad to see a good article about this, and am looking forward to the author's ebook.

Question on the book (if author sees this): will the book cover the npm V8 wrapper Nan (https://www.npmjs.com/package/nan)?

[freezer333 → wasdfan]

Author here... yes! Full table of contents is here http://scottfrees.com/ebooks/nodecpp/ - Chapter 7 will cover using nan.

[amelius]

http://www.swig.org also supports V8.

Does anybody know if NodeJS uses SWIG?

[wasdfan → amelius]

Swig is amazing at quickly converting simple functions (i.e., functions that return an int and other simple types). But if you want to make a C++ function that returns an array or something like a JS object, SWIG doesn't seem to be able to automatically convert it into its javascript equivalent (AFAIK -- could be wrong here).

[catnaroek]

Can Node addons be used to finally provide a workable integer datatype for JavaScript?