Transform Elm Debug.log output into nice log object with custom formatter

elm, debug, console, parser, formatter
npm install elm-debug-transformer@1.1.0-dev


Elm Debug Transformer


Transform Elm Debug.log output into nice log object with custom formatter

The standard Elm Debug.log console output:

Elm Debug.log in console without formatter

and the same output with this package

Elm Debug.log with this package and custom formatter enabled in Chrome

The main module exposes register() function that replaces your console.log() and try to parse each incoming message with Elm parser. If it fails, it would pass the original message.


Right now you can insert only alphabet characters and spaces as a Debug.log tag.

-- this would parse successfuly
Debug.log "Some tag string" thingToPrintToConsole

-- this would NOT BE PARSED
Debug.log "Some String (with non [a-zA-Z] chars or numbers in it) " thingToPrintToConsole

This limitation is due to the problem recognizing arbitrary tag text from the rest of the types. I'm aware of that limitation and it is something that would be addressed in the upcoming versions. Thanks for understanding.


Just install this module with:

yarn add -D elm-debug-transformer


Register the console debugger in your main JS file before you initialize Elm application:

import * as ElmDebugger from 'elm-debug-transformer';


// rest of application

Enable custom formatters in Chrome dev tools

Available in Chrome 47 and higher.

The output object is kind of chatty right now (it carries information about parsed type etc. - less verbose version is worked on right now).

If your browser have Chrome dev toools, you can enable custom formatters so you get less noice and nice output.

  • Open DevTools
  • Go to Settings ("three dots" icon in the upper right corner of DevTools > Menu > Settings F1 > Preferences > Console)
  • Check-in "Enable custom formatters"
  • Close DevTools
  • Open DevTools

Note: You might need to refresh the page first time you open Console panel with existing logs - custom formatters are applied only to newly printed console messages.

Simple object output

import * as ElmDebugger from 'elm-debug-transformer';

ElmDebugger.register({simple_mode: true});

If you are not a fan of Chromium based browser you can pass option to the register function.

register({simple_mode: true});

That way the Debug.log would output simpler JS object without type information. Tuple, Set, Array and List would become arrays and Dict would become JS object with keys and values.


This would probably not see the light of the day without Matt Zeunert and his blogpost about writing custom formatters. Thank you!