Exploring the V8 JS engine (Part 1 of 2)

This is the first part of a 2 part series giving a simple technical overview of the V8 Javascript engine.

First of all some basic facts:

V8 is written in C++ so you should have basic understanding of OOP and some C/C++ knowledge wouldn't do bad too.

The engine (V8) executes Javascript in so called contexts, these are sandboxed and you can create multiple contexts in one V8 virtual machine (engine).

This has the advantage that if you run 2 (or more) Javascript programs you don't have to worry about namespacing. The creation of these contexts is not as memory hungry as you may think, so don't worry about that.

One of the great features of V8 is sharing C++ functions, objects and variables with Javascript, which we will cover in the second part of this series. To use V8 you write a C++ program that uses the V8 libs (to set up the contexts, scopes, templates) and then executes a string which is your Javascript program. You will understand how this works later in this post.

I'll take you through setting up the V8 lib and a simple Hello World! program.

Downloading and Building

I'm using a Mac to build the library, but .

To build V8 you will need:

To download the source type this into you terminal:


$ mkdir ~/dev/
$ cd ~/dev/
$ svn checkout http://v8.googlecode.com/svn/trunk/ v8-read-only
$ mv v8-read-only v8

Now you have the source you can build the V8 library and header file:


$ scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
...

Simple isn't it? Now once the build has finished you should be able to see the libv8.a file. This file is important for compiling the C++ program that uses the V8 library.

Your first V8 program

Now that you have all the parts needed for compiling a C++ program that uses the V8 classes, let's get to the interesting part.

Here is a simple C++ program that runs the Javascript: "Hello World!". Obviously, this is not a very spectacular Javascript program, but it should do to our needs. The comments explain what a each part does:


#include 

using namespace v8;

int main(int argc, char* argv[]) {

  // Create a new context.
  Persistent<Context> context = Context::New();

  // Enter the created context for compiling and
  // running the hello world program.
  context->Enter();

  // Create a stack-allocated handle scope.
  HandleScope handle_scope;

  // Create a string containing the JavaScript code
  // to execute (notice the quotation).
  Handle<String> source = String::New(" 'Hello World!'; ");

  // Compile the Javascript code.
  Handle<Script> script = Script::Compile(source);

  // Run the script to get the result.
  Handle<Value> result = script->Run();

  // Get rid of the persistent context.
  context.Dispose();

  // Convert the result to an ASCII string and print it.
  String::AsciiValue ascii(result);
  printf("%s\n", *ascii);

  return 0;
}

Save that in a file called hello_world.cc in the ~/dev/v8/ directory.

Next you need to compile the hello_world.cc file by doing:


$ g++ -m32 -Iinclude libv8.a -lpthread hello_world.cc -o hello_world

This compiles the hello world program to a 32-bit executable that includes the libv8.a library.

If the compilation was successful you should have a new file called hello_world in your V8 directory. Then you execute the file and it should print out "Hello World!" (without the quotes):


$ ./hello_world
Hello World!

Congratulations you have just run your first program that uses V8.

In the next part of this series we'll look into sharing variables and objects and dig into the source of Node.js.