Calculate difference in percentage between 2 hex colors

colormeter.js

function color_meter(cwith, ccolor) {
    
    if (!cwith && !ccolor) return;
    
    var _cwith  = (cwith.charAt(0)=="#") ? cwith.substring(1,7) : cwith;
    var _ccolor = (ccolor.charAt(0)=="#") ? ccolor.substring(1,7) : ccolor;
    
    var _r = parseInt(_cwith.substring(0,2), 16);
    var _g = parseInt(_cwith.substring(2,4), 16);
    var _b = parseInt(_cwith.substring(4,6), 16);

    var __r = parseInt(_ccolor.substring(0,2), 16);
    var __g = parseInt(_ccolor.substring(2,4), 16);
    var __b = parseInt(_ccolor.substring(4,6), 16);
    
    var p1 = (_r / 255) * 100;
    var p2 = (_g / 255) * 100;
    var p3 = (_b / 255) * 100;

    var perc1 = Math.round((p1 + p2 + p3) / 3);
    
    var p1 = (__r / 255) * 100;
    var p2 = (__g / 255) * 100;
    var p3 = (__b / 255) * 100;
    
    var perc2 = Math.round((p1 + p2 + p3) / 3);
    
    return Math.abs(perc1 - perc2);   
}

When running this script on http://js.do this line is given:

JavaScript error: TypeError: cwith.charAt is not a function on line 9

You can see the used code in http://js.do/code/color_meter-test-0

Python 2.7+ translation: https://gist.github.com/rautamiekka/989d644a4982a73b4919516d8dad39dc

@rautamiekka - it needs to be a string.

color_meter('#6D161C','#E45E69');

works great 👍 thanks !!

I know this is really old, but I found it helpful. Put this into a plunkr to make it easier to test out in practice, if needed.

https://plnkr.co/edit/Yi9mApupEbA6dbml?p=preview