Chrome Debugger

GraalVM supports debugging of guest language applications and provides a built-in implementation of the Chrome DevTools Protocol. This allows you to attach compatible debuggers such as Chrome Developer Tools to GraalVM.

To debug guest language applications, pass the --inspect option to the command line launcher, as in the following example with a Node.js HelloWorld program:

var http = require('http');

var server = http.createServer(function (request, response) {
  response.writeHead(200, {"Content-Type": "text/plain"});
  response.end("Hello World!\n");


console.log("Server running at http://localhost:8000/");
  1. Save this program as HelloWorld.js and then run:
    node --inspect --jvm HelloWorld.js
    Debugger listening on port 9229.
    To start debugging, open the following URL in Chrome:
    Server running at http://localhost:8000/
  2. Navigate to http://localhost:8000/ in your browser to launch the node application.

  3. Open the chrome-devtools:... link in a separate Chrome browser tab.

  4. Navigate to the HelloWorld.js file and submit a breakpoint at line 4.

  5. Refresh the node.js app and you can see the breakpoint hit.

Now you can inspect the stack, variables, evaluate variables, and selected expressions in a tooltip, and so on. By hovering your cursor over the response variable, for instance, you can inspect its properties, as seen in the screenshot below:

Consult the JavaScript Debugging Reference for details on Chrome DevTools debugging features.

This debugging process applies to all guest languages that GraalVM supports. Other languages such as R and Ruby can be debugged as easily as JavaScript, including stepping through language boundaries during guest language interoperability.

Inspect Options

Node Launcher

The Node.js runtime of GraalVM accepts the same options as node.js built on the V8 JavaScript engine, such as:

--inspect[=[host:]<port number>]

This enables the inspector agent and listens on port 9229 by default. To listen on a different port, specify the optional port number:

--inspect-brk[=[host:]<port number>]

This applies to the node launcher only.

Other Language Launchers

Other guest language launchers such as js, python, Rscript, ruby, lli, and polyglot accept the --inspect[=[host:]<port number>] option, but suspend on the first line of the application code by default.


This disables the initial suspension if you specify --inspect.Suspend=false.

Additional Common Inspect Options

All launchers also accept the following additional options:

Advanced Debug Options

The following options are for language experts and language developers:

Programmatic Launch of Inspector Backend

Embedders can provide the appropriate inspector options to the Engine/Context to launch the inspector backend. The following code snippet provides an example of a possible launch:

import org.graalvm.polyglot.*;

class DebuggerSample {
    public static void main(String... args) {
        String port = "4242";
        String path = java.util.UUID.randomUUID().toString();
        Context context = Context.newBuilder("js")
                    .option("inspect", port)
                    .option("inspect.Path", path)
        String hostAdress = "localhost";
        String url = String.format(
                    hostAdress, port, path);