Category Archives: Javascript

Edge JsRT: Object Before Collection Callbacks

There are a number of new features in the Edge version of JsRT, and since I haven’t seen anything discussing them on the web, I thought I’d walk through them.

The first one I thought I’d discuss is object before collection callbacks. In the first version of JsRT, you could create an “external” object that could have a callback that would be called when the object was finalized. But this only allowed you to know about objects that you create. What if you need to know when an object that you’ve attached some special significance to is collected? Now you can call JsSetObjectBeforeCollectCallback, provide it the reference to track, some state, and a callback function. When the GC is going to collect the object, you’ll get a callback to let you know.

Useful for managing lifetime of native objects that track some JS object!

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The Two Faces of JsRT (in Windows 10)

In terms of Windows 10, probably the biggest thing to happen to the JsRT APIs in Windows was the IE/Edge browser split.

For anyone who’s been living under a rock, Microsoft introduced a new browser in Windows 10 called Edge (neé Project Spartan). Edge basically represents a fork in the road for the web browser codebase at Microsoft: down one path the IE code will continue on, largely unchanged and preserving all that crufty backwards compatibility we all love to hate, and down another path will go Edge, discarding all the legacy shackles of IE and committing to a thoroughly modern browser experience.

And, just as the browser now splits into IE and Edge, so does the underlying JavaScript engine, Chakra. There are now two (yes, two!) JsRT APIs:

  • The “legacy” JsRT APIs that are exposed by jscript9.dll (the one that IE uses), have not changed since Windows 8.1, and will not be added to.
  • The “Edge” JsRT APIs that are exposed by chakra.dll (the one that Edge uses), have some breaking changes since Windows 8.1, and will be updated as the Edge browser moves forward.

You can find more details on the breaking changes that were made to the Edge JsRT on MSDN, but in a nutshell they are:

  • In C++ you #define USE_EDGEMODE_JSRT before including jsrt.h, and you link against chakrart.lib instead of jsrt.lib.
  • In C# and VB, you target your PInvoke declarations at chakra.dll instead of jscript9.dll.
  • The “version” parameter of JsCreateRuntime disappears (since there will only ever be one evergreen version of the Edge Chakra).
  • The debugging no longer requires an IDebugApplication. Thus, JsStartDebugging and JsCreateContext lose a parameter.

The last bullet point may seem a little odd and arbitrary. The reason is that the Edge version of the JsRT API is now supported as a part of the UWP (Universal Windows Platform). That means that a “modern” UWP application can host the Edge Chakra runtime and still qualify for the Windows Store (i.e. it won’t be rejected for using non-UWP APIs). To get there, though, the Chakra team needed to remove things that don’t work on the UWP, namely talking to the PDM (Process Debug Manager), which is what you need to get an IDebugApplication. Since it turns out in almost every possible case you don’t need to be able to supply your own IDebugApplication, Chakra now just acquires one on your behalf.

I’ve updated my chakra-host sample on GitHub to include samples that show hosting both “legacy” and “edge” JsRT on Windows 10. I’ve also added a new sample to the Windows Samples showing this. The Chakra team also now has a GitHub repo where they host their own samples.

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Running node.js on Chakra!

It’s been a very busy year since I last posted, and a lot has been happening. As a result, I’ve kind of fallen behind a bit on all the stuff that came out around the Windows 10 launch. And one of the biggest things that I missed blogging about was the revelation of a (temporary) fork of node.js that runs Node on Chakra instead of V8!

This was actually the culmination of some work that I started several years ago as an internal project to enable some teams at Microsoft to author node.js applications that ran on Chakra instead of V8. The problem, then as now, was that there were some platforms where V8 didn’t or couldn’t run. So I spent a fun month or so working with another team concocting a shim over the nascent JSRT APIs that made them look a lot like the V8 APIs. The end result was a node.js binary that used Chakra and ran all the stuff that the team I was working with needed. Then the code went sort of into hibernation, and when I left the Chakra team, my expectation was that nothing more would come of it.

But, lo and behold, the IoT folks wanted node.js and they wanted it on ARM, which V8 doesn’t have an official code generator for, so that code miraculously got a second chance at life! After a lot of extra work on the part of the Chakra team (since the original shim only covered the scenarios the internal team needed), it was polished up and is now running out in the wild. It’s great to see that work continuing, especially since node.js can really be a joy to program in. Combining node.js with the ability of Chakra to access UWP (Universal Windows Platform) APIs also opens intriguing possibilities.

I’ve been thinking that I wanted to clone the fork to see if there was anything I could contribute, but things have been just too busy. Soon, maybe!

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NodeRT: Accessing WinRT from Node.js

If you’re using node.js on Windows and are interested in getting easy access to some of the cool WinRT-based APIs in Windows 8.0 and later, you should check out the new NodeRT project on Github. It’s a tool that allows you to automagically generate native node.js modules from WinRT winmd metadata files, making them easily available in node:

Since node.js runs as a regular Win32 application and not a Windows Store application (is that what we’re calling Metro these days?), you won’t be able to get access to Store-only WinRT APIs, but there are still quite a few useful WinRT APIs available to desktop apps.

Pretty awesome, check it out!

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JsRT: Wrappers updated to v1.1

Even though I may not be working on the Chakra team any more, I’m still doing some noodling on making the hosting APIs easier to use. I’ve got some other interesting things coming soon, but on the way there I made some improvements/fixes to the wrappers that I released a while back.

Changes to v1.1:

  • PDB files are generated along with the LIB, and debug builds are included. All flavors are now packaged together.
  • Header file name is changed to jsrt-wrappers.h to be consistent with the other files.
  • Added equality operators for handles.
  • Added function_base::create which can deduce the strongly typed function wrapper from the function signature.
  • Fixes the handling of optional parameters in functions.
  • Changes the handling of rest parameters to use std::vector rather than a JavaScript array, which is more efficient.
  • Simplified prototype handling.
  • Bug fixes.

Let me know if there are any problems!

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JsRT: Wrappers for C++

One unfortunate side-effect of the fact that the JsRT API is a flat Win32-style API is that it can be a little… painful to use in C++. Some of the conventions, particularly the use of error code returns everywhere and the use of return parameters, mean that you have to write a lot of boilerplate code. And marshalling values between C++ and JavaScript can be very cumbersome. So with that in mind, I’ve written some sample C++ wrappers that make the process of working with the JsRT APIs a lot nicer in C++.

You can get them from my GitHub account here. For more information, here’s part of the readme, which covers some of the highlights of the wrappers:

A Tour Through the Helpers

Most of the wrappers simply provide a nice object-oriented interface over the JsRT API. For example, creating a runtime and a context works like this:

jsrt::runtime runtime = jsrt::runtime::create();
jsrt::context context = runtime.create_context();

There are, however, a number of special features in the wrappers.

Context scopes

A context scope (jsrt::context::scope) automatically sets the current context when it comes into scope and clears the current context when it goes out of scope. This greatly simplifies working with scopes. For example:

jsrt::runtime runtime = jsrt::runtime::create();
jsrt::context context = runtime.create_context();
    jsrt::context::scope scope(context);
    ... work with context ...
// Context has automatically been set back to original value.

Pinned references

To keep a reference to a Chakra object alive, it must be visible to the Chakra garbage collector (i.e. on the stack or stored in the garbage collected heap) or it has to be pinned using JsAddRef. The pinned<T> class functions as a smart-pointer template that keeps a reference alive using JsAddRef and JsRelease automatically. For example:

    jsrt::pinned<jsrt::object> pinned_obj = jsrt::object::create();
    // Object reference does not have to stay on the stack or in the GC heap
// Reference can now be garbage collected

Value translation

Value getter and setter functions can be strongly-typed, allowing automatic marshalling of JavaScript values to C++ values and vice versa. For example:

jsrt::object obj = jsrt::object::create();
obj.set_property(jsrt::property_id::create(L"boolProperty"), true);
bool bool_value = obj.get_property<bool>(jsrt::property_id::create(L"boolProperty"));
obj.set_property(jsrt::property_id::create(L"stringProperty"), L"foo");

The wrappers marshal types in the following way:

  • number values are marshalled to/from double

  • string values are marshalled to/from std::wstring

  • Boolean values are marshalled to/from bool

Strongly-typed arrays

Similarly, arrays can be strongly typed and accessed using numeric indexes:

jsrt::array<double> darray = jsrt::array<double>::create(1);
darray[0] = 10;
darray[1] = 20;


Perhaps the most useful aspect of the wrappers is the way that JavaScript functions can be wrapped and their arguments and return values marshalled to C++ types, and vice versa. For example:

auto f = (jsrt::function<double, double, double>)jsrt::context::evaluate(
    L"function f(a, b) { return a + b; }; f;");
double a = f(jsrt::context::undefined(), 1, 2);

Native functions can also be wrapped:

double add(const jsrt::call_info &info, double a, double b) { return a + b; }
auto nf = jsrt::function<double, double, double>::create(add);
jsrt::context::global().set_property(jsrt::property_id::create(L"add"), nf);
jsrt::context::run(L"add(1, 2)");

When calling a function wrapper, the first argument to the function call is the this value for the call. Function wrappers can also be bound to a particular value of this. For example:

auto bf = jsrt::bound_function<jsrt::value, double, double, double>::create(
    (jsrt::function<double, double, double>)jsrt::context::evaluate(
        L"function f(a, b) { return a + b; }; f;"));
double ba = bf(1, 2);

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Using Jake with TypeScript

I’ve been playing around a lot lately with TypeScript and node.js, and have been using Jake as my make system. Oddly, I haven’t been able to find any npms that simplify Jake/TypeScript integration (maybe I’m not looking hard enough?), so I wrote a little one of my own, jake-typescript. It exposes two methods:

  • batchFiles takes a list of files and produces a Jake task that compiles each of the files separately (i.e. the default tsc behavior).
  • singleFile takes a list of files and produces a Jake file task that compiles all of the files into a single .js file (i.e. the –out behavior).

There’s also a structure to specify command-line arguments for the compile. It’s my first npm and I’m still getting up to speed on all the various conventions, definitely looking for feedback…

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