Add Styles to Console Statements

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I was recently checking out Google Plus because they implement some awesome effects.  I opened the console and same the following message:

WARNING!

Using this console may allow attackers to impersonate you and steal your information using an attack called Self-XSS.

Do not enter or paste code that you do not understand.

I wasn't surprised to see that message but what I did notice was that the text was red and the background was yellow.  The text was even a bit bigger.  How did they do it?  Pretty easily:

console.log("%c%s",
            "color: red; background: yellow; font-size: 24px;",
            "WARNING!");

The first argument is the order of style and message, the second is the style set, and the last is the desired message.

As to why you'd want to use this?  If it helps you identify debug information easier in the console, you may consider calling more attention to some messages!

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Discussion

  1. Easy, if one controls the browser and therefor the console, that has to display this ;) But Firefox supports this as well, and only IE prints both, the format string as the string itself 1:1 into F12.

  2. There’s a JS library that makes styling comments much easier:

    https://github.com/astoilkov/console.message

  3. By replacing your formatting placeholders with %c%o, it will output a readable repr for any type of object you pass in, not just the str value.

    https://jsfiddle.net/jsatt/Lta6p2jz/2/

  4. very cool advice! my console is very colour ;)

  5. With some trickery you can inject images too!

    console.log("%cDuckie.TV", "color:transparent; font-size: 16pt; line-height: 125px; padding:25px; padding-top:30px; padding-bottom:60px; background-image:url(http://duckietv.github.io/DuckieTV/img/icon128.png); background-repeat:no-repeat; ", "quack!\n\n\n\n\n\n");
    
  6. Amelia BR

    I find it amusing that this post starts with an example of a warning not to copy & paste code into the console, and ends with me wanting to copy & paste code from people’s comments into the console.

    But it’s okay, cause I sort of understand it, right?

  7. schadeck

    I like to put an object at the top of my script when I am developing to create a shortcut for styling my logs:

    // for styling console.log
    // EXAMPLE: console.log(sty.sty, sty.red, 'thingToLog:' ,+ foo);
    var sty = {
        sty: '%c%s',
        grn: 'background:#9c9;color:#141;padding:.25em;line-height:1.5em;',
        ylw: 'background:#ff0;color:#993;padding:.25em;line-height:1.5em;',
        red: 'background:#c99;color:#411;padding:.25em;line-height:1.5em;',
        org: 'background:#da0;color:#830;padding:.25em;line-height:1.5em;',
        gry: 'background:#666;color:#fff;padding:.25em;line-height:1.5em;',
        prp: 'background:#97e;color:#405;padding:.25em;line-height:1.5em;',
        blu: 'background:#8ac;color:#036;padding:.25em;line-height:1.5em;'
    };
    

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!