What is the equivalent of Java's System.out.println() in Javascript?

Essentially console.log("Put a message here.") if the browser has a supporting console.

Another typical debugging method is using alerts, alert("Put a message here.")

RE: Update II

This seems to make sense, you are trying to automate QUnit tests, from what I have read on QUnit this is an in-browser unit testing suite/library. QUnit expects to run in a browser and therefore expects the browser to recognize all of the JavaScript functions you are calling.

Based on your Maven configuration it appears you are using Rhino to execute your Javascript at the command line/terminal. This is not going to work for testing browser specifics, you would likely need to look into Selenium for this. If you do not care about testing your JavaScript in a browser but are only testing JavaScript at a command line level (for reason I would not be familiar with) it appears that Rhino recognizes a print() method for evaluating expressions and printing them out. Checkout this documentation.

These links might be of interest to you.

QUnit and Automated Testing

JavaScript Unit Tests with QUnit

I found a solution:

print("My message here");

I'm using Chrome and print() literally prints the text on paper. This is what works for me:

document.write("My message");