Evaluating a String of JS

Using View::EvaluateScript()

The easiest way to evaluate a string of JavaScript in C++ is by using View::EvaluateScript()

// Use LoadListener::OnDOMReady to wait for page scripts to load 
void MyApp::OnDOMReady(View* view) {
  
  // Execute a string of JavaScript
  view->EvaluateScript("document.body.innerHTML = 'Ultralight rocks!';");
  
}

Handling Results

View::EvaluateScript() can return the result from a script execution, if any.

The result is returned in the form of a JSValueRef which you can use with JavaScriptCore's API to convert into native C types.

// Use LoadListener::OnDOMReady to wait for page scripts to load 
void MyApp::OnDOMReady(View* view) {
  
  // Execute a string of JavaScript, store result in 'result
  JSValueRef result = view->EvaluateScript("1 + 1");
  
  // Convert result to a Number ('double' in C)
  double num = JSValueToNumber(view->js_context(), result, 0);
  
  // 'num' should now equal '2.0'
  
}

Using JSEvaluateScript() Directly

View::EvaluateScript() is really just a wrapper for JavaScriptCore's JSEvaluateScript(). This API function offers lower-level control over script execution (exception handling, overriding this object, etc.).

Read the API declaration for JSEvaluateScript() here.

To use this API function we're going to need to create a JavaScript string with JSStringCreateWithUTF8CString() and then pass it to JSEvaluateScript().

// Use LoadListener::OnDOMReady to wait for page scripts to load 
void MyApp::OnDOMReady(View* view) {
  
  // Get the JS execution context for the current page
  JSContextRef ctx = view->js_context();
  
  const char* script = "document.body.innerHTML = 'Ultralight rocks!';";
  
  // Create our string of JavaScript
  JSStringRef str = JSStringCreateWithUTF8CString(script);
  
  // Execute it with JSEvaluateScript, ignoring other parameters for now
  JSEvaluateScript(ctx, str, 0, 0, 0, 0);
  
  // Release our string (we only Release what we Create)
  JSStringRelease(str);
  
}

Managing string lifetime with Retain / Release

The API follows a simple rule-- anything you "Create" you must "Release".

For example, in the above code snippet we called JSStringCreateWithUTF8CString. This API function has the word Create in it so we must release the created object with JSStringRelease() to avoid a memory leak.

If you want to increase the ref-count (to retain the object in a C++ wrapper, for instance), you can call JSStringRetain().

The API provides a JSRetainPtr<> for C++ that automatically manages lifetime for you. Just call adopt() with any of the JSStringCreate methods to create a managed string object.

#include <JavaScriptCore/JSRetainPtr.h>

// Use LoadListener::OnDOMReady to wait for page scripts to load 
void MyApp::OnDOMReady(View* view) {
  
  // Get the JS execution context for the current page
  JSContextRef ctx = view->js_context();
  
  const char* script = "document.body.innerHTML = 'Ultralight rocks!';";
  
  // Create our string of JavaScript, automatically managed by JSRetainPtr
  JSRetainPtr<JSStringRef> str = adopt(
    JSStringCreateWithUTF8CString(script));
  
  // Execute it with JSEvaluateScript, ignoring other parameters for now
  JSEvaluateScript(ctx, str.get(), 0, 0, 0, 0);
  
}