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Starting the REPL

Noah Isaacson edited this page · 5 revisions

Go to Firefox Tools menu and select MozLabStart MozRepl. MozRepl is now listening for connections on the default port 4242.

Open the command shell of your operating system and type:

    telnet localhost 4242

You’ll be presented with the prompt:


You’re in!

Platform-specific notes

If you’re using Microsoft’s telnet for Windows, it’s probably set to handle one-character-at-a-time instead of one-line-at-a-time. See the Troubleshooting page, or just use netcat (recommended) or putty.

If you’re on Linux or some other Unix variant, you can use rlwrap to get command history and other niceties. Use arrow-up/down to retrieve previous commands and Control+R to search among them.

    $ rlwrap telnet localhost 4242

If you’re using Emacs, see the Emacs integration page.

Starting on a specific port

To start the repl via the command line on a specific port, execute the following command depending on your platform.


/Applications/ -profile /path/to/profile/folder -repl -repl 7070
/usr/bin/firefox -profile /path/to/profile/folder -repl -repl 7070

The profile path in the examples above must be a profile that has the mozrepl extension installed.

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