A function that's using https://github.com/kpdecker/jsdiff to display a difference between two strings in a console. Tested on latest versions of Firebug and Chrome Developer Tools. Developer tools in IE 10 and below don't support CSS styles in console.

consoleDiff.js

(function() {
    // A function that's using https://github.com/kpdecker/jsdiff to display a 
    // difference between two strings in a console.
    // Tested on latest versions of Firebug and Chrome Developer Tools.
    // Developer tools in IE 10 and below don't support CSS styles in console.
    var consoleDiff = function (first, second, diffType) {
        var normalStyle = 'color: black; background-color: white; text-decoration: none;',
            addStyle = 'color: #406619; background-color: #eaf2c2; text-decoration: none;',
            removeStyle = 'color: #b30000; background-color: #fadad7; text-decoration: line-through;',
            text = '',
            arguments = [],
            diff;
        
        if (typeof JsDiff === 'undefined') {
            throw new Error('jsdiff script not loaded. ' +
                'Download diff.js from: https://github.com/kpdecker/jsdiff ' +
                'and include it on the page.');
        }
        
        if (typeof first === 'function') {
            first = first();
        }
        if (typeof second === 'function') {
            second = second();
        }
        
        // diffType can be: diffChars, diffWords and diffLines
        diffType = diffType || 'diffWords'; 

        diff = JsDiff[diffType](first, second);
        
        for (var i = 0; i < diff.length; i++) {
            
            // Removed text should in front of added
            if (diff[i].added && diff[i + 1] && diff[i + 1].removed) {
                var swap = diff[i];
                diff[i] = diff[i + 1];
                diff[i + 1] = swap;
            }

            text = text.concat('%c', diff[i].value);
            
            if (diff[i].removed) {
                arguments.push(removeStyle);
            } else if (diff[i].added) {
                arguments.push(addStyle);
            } else {
                arguments.push(normalStyle);
            }
        }

        arguments.unshift(text);
        console.log.apply(console, arguments);
    };

    // Sample 1: simple strings
    consoleDiff('My first string', 'My second string');
    
    // Sample 2: more complex strings (JSON) 
    consoleDiff(
        '{"employeeNumber":101,"ssn":"123-45-6789","firstName":"Bruce","middleInitial":"",' +
            '"lastName":"Wayne","address1":"140 Fell Court","address2":"","city":"Gotham",' +
            '"state":"NY","zip":"10000"}',
        '{"employeeNumber":101,"ssn":"123-45-6789","firstName":"Alfred","middleInitial":"",' +
            '"lastName":"Wayne","address1":"142 Fell Court","address2":"","city":"Gotham",' +
            '"state":"NY","zip":"10001"}'
    );
    
    // Sample 3: using functions as arguments (they need to return a string)
    consoleDiff(
        function () { return 'First function result' },
        function () { return 'Second function result' }
    );
    
    // Sample 4: using char diff
    consoleDiff('abcdefghijklmnopqrstuvwxyz', 'abcdefghijklabcopqrstuvwxyz', 'diffChars');
    
    // This utility is created for debugging ko.dirtyFlag
    // One ko.dirtyFlag implementation can be found on:
    // http://www.knockmeout.net/2011/05/creating-smart-dirty-flag-in-knockoutjs.html
    // It's easy to add a method to it to log the differences between initial and 
    // current state somewhere in dirtyFlag code, like this
    result.log = function() {
        consoleDiff(_initialState(), ko.toJSON(root));
    };
}());