Using the Chrome Devtools JavaScript preprocessing feature

The Chrome Devtools JavaScript preprocessor intercepts JavaScript just before it enters V8, the Chrome JS system, allowing the JS to be transcoded before compilation. In combination with page injected JavaScript, the preprocessor allows a complete synthetic runtime to be constructed in JavaScript. Combined with other functions in the chrome.devtools extension API, the preprocessor allows new more sophisticated JavaScript-related developer tools to be created.


To use the script preprocessor, write a chrome devtools extension that reloads the Web page with the preprocessor installed:

    ignoreCache: true,
    injectedScript: runThisFirst,
    preprocessorScript: preprocessor

where preprocessorScript is source code (string) for a JavaScript function taking three string arguments, the source to preprocess, the URL of the source, and a function name if the source is an DOM event handler. The preprocessorerScript function should return a string to be compiled by Chrome in place of the input source. In the case that the source is a DOM event handler, the returned source must compile to a single JS function.

The Chrome Preprocessor Example illustrates the API call in a simple chrome devtools extension. Download and unpack the .zip file, use chrome://extensions in Developer Mode and load the unpacked extension. Then open or reopen devtools. The Preprocessor panel has a reload button that triggers a simple preprocessor.

The preprocessor runs in an isolated world similar to the environment of Chrome content scripts. A window object is available but it shares no properties with the Web page window object. DOM calls in the preprocessor environment will operate on the Web page, but developers should be cautious about operating on the DOM in the preprocessor. We do not test such operations though we expect the result to resemble calls from the outer function of <script> tags.

In some applications the developer may coordinate runtime initialization using the injectedScript property in the object passed to the reload() call. This is also JavaScript source code; it is compiled into the page ahead of any Web page scripts and thus before any JavaScript is preprocessed.

The preprocessor is compiled once just before the first JavaScript appears. It remains active until the page is reloaded or otherwise navigated. Navigating the Web page back and then forward will result in no preprocessing. Closing devtools will leave the preprocessor in place.

Use Cases

The script preprocessor supports transcoding input source to JavaScript. Use cases include:

  • Adding write barriers for Querypoint debugging,
  • Supporting feature-specific debugging of next generation EcmaScript using eg Traceur,
  • Integration of development tools like coverage analysis.
  • Analysis of call sequences for performance tuning.

Several JavaScript compilers support transcoding, including Traceur and Esprima.


The implementation relies on the Devtools front-end hosting an extension supplying the preprocessor script; the front end communicates with the browser backend over eg web sockets.

The devtools extension function call issues a postMessage() event from the devtools extension iframe to the devtools main frame. The event is handled in ExtensionServer.js which forwards it over the devtools remote debug protocol. (See Bug 229971 for this part of the implementation and its status).

When the preprocessor script arrives in the back end, InspectorPageAgent::reload stores the preprocessor script in m_pendingScriptPreprocessor. After the browser begins the reload operation, it calls PageDebuggerAgent::didClearWindowObjectInWorld which moves the processor source into the scriptDebugServer().

Next the browser prepares the page environment and calls PageDebuggerAgent::didClearWindowObjectInWorld. This function clears the preprocessor object pointer and if it is not recreated during the page load, no scripts will be preprocessed. At this point we only store the preprocessor source, delaying the compilation of the preprocessor until just before its first use. This helps ensure that the JS environment we use is fully initialized.

Source to be preprocessed comes from three different places:

  1. Web page <script> tags,
  2. DOM event-listener attributes, eg onload,
  3. JS eval() or new Function() calls.

When the browser encounters either a <script> tag (ScriptController::executeScriptInMainWorld) or an element attribute script (V8LazyEventListener::prepareListenerObject) we call a corresponding function in InspectorInstrumentation. This function has a fast inlined return path in the case that the debugger is not attached.

If the debugger is attached, InspectorInstrumentation will call the matching function in PageDebuggerAgent (see core/inspector/InspectorInstrumentation.idl). It checks to see if the preprocessor is installed. If not, it returns.

The preprocessor source is stored in PageScriptDebugServer. If the preprocessor is installed, we check to see if it is compiled. If not, we create a new ScriptPreprocessor object. The constructor uses ScriptController::executeScriptInIsolatedWorld to compile the preprocessor in a new isolated world associated with the Web page's main world. If the compilation and outer script execution succeed and if the result is a JavaScript function, we store the resulting function as a ScopedPersistent<v8::Function> member of the preprocessor.

If the PageScriptDebugServer::preprocess() has a value for the preprocessor function, it applies the function to the web page source using V8ScriptRunner::callAsFunction(). This calls the compiled JS function in the ScriptPreprocessor's isolated world and retrieves the resulting string.

When the preprocessed JavaScript source runs it may call eval() or new Function(). These calls cause the V8 runtime to compile source. Immediately before compiling, V8 issues a beforeCompile event which triggers ScriptDebugServer::handleV8DebugEvent(). This code is only called if the debugger is active. In the handler we call ScriptDebugServer::preprocessEval() to examine the ScriptCompilationTypeInfo, a marker set by V8, to see if we are compiling dynamic code. Only dynamic code is preprocessed in this function and only if we are not executing the preprocessor itself.

During the browser operation, API generation code, debugger console initialization code, injected page script code, debugger information extraction code, and regular web page code enter this function. There is currently no way to distinguish internal or system code from the web page code. However the internal code is all static. By limiting our preprocessing to dynamic code in the beforeCompile handler, we know we are only operating on Web page code. The static Web page code is preprocessed as described above.


We currently do not support preprocessing of WebWorker source code.