gecko-dev/addon-sdk/source/doc/dev-guide-source/tutorials/logging.md

Logging

To follow this tutorial you'll need to have [installed the SDK](dev-guide/tutorials/installation.html) and learned the [basics of `cfx`](dev-guide/tutorials/getting-started-with-cfx.html).

The DOM console object is useful for debugging JavaScript. Because DOM objects aren't available to the main add-on code, the SDK provides its own global console object with most of the same methods as the DOM console, including methods to log error, warning, or informational messages. You don't have to require() anything to get access to the console: it is automatically made available to you.

The console.log() method prints an informational message:

console.log("Hello World");

Try it out:

• create a new directory, and navigate to it
• execute cfx init
• open "lib/main.js" and add the line above
• execute cfx run, then cfx run again

Firefox will start, and the following line will appear in the command window you used to execute cfx run:

info: Hello World!

console in Content Scripts

You can use the console in content scripts as well as in your main add-on code. The following add-on logs the HTML content of every tab the user loads, by calling console.log() inside a content script:

require("sdk/tabs").on("ready", function(tab) {
  tab.attach({
    contentScript: "console.log(document.body.innerHTML);"
  });
});

console Output

If you are running your add-on from the command line (for example, executing cfx run or cfx test) then the console's messages appear in the command shell you used.

If you've installed the add-on in Firefox, or you're running the add-on in the Add-on Builder, then the messages appear in Firefox's Error Console.

But note that by default, calls to console.log() will not result in any output in the Error Console for any installed add-ons: this includes add-ons installed using the Add-on Builder or using tools like the Extension Auto-installer.

See ["Logging Levels"](dev-guide/console.html#Logging Levels) in the console reference documentation for more information on this.

Learning More

For the complete console API, see its API reference.