Getting Started

This document introduces some key V8 concepts and provides a hello world example to get you started with V8 code.

Contents

Audience

This document is intended for C++ programmers who want to embed the V8 JavaScript engine within a C++ application.

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Hello World

Let's look at a Hello World example that takes a JavaScript statement as a string argument, executes it as JavaScript code, and prints the result to standard out.

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

  // Create a string containing the JavaScript source code.
  String source = String::New("'Hello' + ', World'");

  // Compile the source code.
  Script script = Script::Compile(source);
  
  // Run the script to get the result.
  Value result = script->Run();

  // Convert the result to an ASCII string and print it.
  String::AsciiValue ascii(result);
  printf("%s\n", *ascii);
  return 0;
}

To actually run this example in V8, you also need to add handles, a handle scope, and a context:

• A handle is a pointer to an object. All V8 objects are accessed using handles, they are necessary because of the way the V8 garbage collector works.
• A scope can be thought of as a container for any number of handles. When you've finished with your handles, instead of deleting each one individually you can simply delete their scope.
• A context is an execution environment that allows separate, unrelated, JavaScript code to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.

The following example is the same as the one above, except now it includes handles, a context, and a scope - it also includes a namespace and the v8 header file:

#include <v8.h>

using namespace v8;

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

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

  // Create a new context.
  Persistent<Context> context = Context::New();
  
  // Enter the created context for compiling and
  // running the hello world script. 
  Context::Scope context_scope(context);

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

  // Compile the source code.
  Handle<Script> script = Script::Compile(source);
  
  // Run the script to get the result.
  Handle<Value> result = script->Run();
  
  // Dispose 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;
}

Handles, garbage collection, contexts, and scopes are discussed in greater detail in the Embedder's Guide.

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Run the Example

Follow the steps below to run the example yourself:

1. Download the V8 source code and build V8 by following the download and build instructions.
2. Copy the complete code from the previous section (the second code snippet), paste it into your favorite text editor, and save as hello_world.cpp in the V8 directory that was created during your V8 build.
3. Compile hello_world.cpp, linking to the libv8.a library created in the build process. For example, on Linux using the GNU compiler:
   g++ -Iinclude hello_world.cpp -o hello_world libv8.a -lpthread
4. Run the hello_world executable file at the command line.
   For example, on Linux, still in the V8 directory, type the following at the command line:
   ./hello_world
5. You will see Hello, World!.

Of course this is a very simple example and it's likely you'll want to do more than just execute scripts as strings! For more information see the Embedder's Guide.

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