Use jQuery to find the shortest selector for a given element in the DOM.

pathfinder.js

jQuery(function($) {
    // returns an array of the potential selector components for the first element in the jQuery object.  IDs, classes, and tagNames only.
    var getSelectorComponents = function($el) {
        var components = [];
        
        var id = $el.attr('id');
        if (typeof(id)!='undefined' && /[^\s]/.test(id)) {
            components.push('#'+id);
        }

        var classes = $el.attr('class');
        if (typeof(classes)!='undefined' && /[^\s]/.test(classes)) {
            classes = '.' + classes.split(/\s+/).join(' .');
            components = components.concat( classes.split(' ') );
        }

        components.push($el[0].tagName);
        return components;
    };
    var getSelectorFromComponents = function(components) {
        return components[components.length-1] + components.slice(0, -1).join('');
    };

    $(document.body).on('click', function(event) {
        event.preventDefault();

        // handle deselection of any previously selected elements
        // (for now, only allows selection of one element at a time)
        var selectedTag = 'cdo-selected';
        $('.'+selectedTag).removeClass(selectedTag).css('box-shadow', 'none');

        var $target = $(event.target).css('box-shadow', '0 0 0 4px rgba(255,100,0,.5)');

        // 2 step process

        // Step 1 - find the closest unique context selector, i.e. `#main .special-container ul`
        // Step 2 - invent a creative selector within that context (if necessary), i.e. `li:nth-child(2) a`

        // Step 1.
        // path will be an array of arrays. for each level of the DOM hierarchy between the body and our target element (inclusive),
        // there will be an array containing the potential selector components we can use for that element
        // i.e. for this container within the DOM path: <div id="main" class="column left blue"></div>
        // we will have this element in our path array: ['#main', '.column', '.left', '.blue', 'DIV']
        // from which we can construct various selectors to test for uniqueness
        // NOTE: because the jQuery .parents() method naturally returns the element ancestors ordered from the inside out, the path array is reversed from the norm
        var path = [];
        $target.parents().each(function() {
            path.push( getSelectorComponents( $(this) ) );
        });
        path.splice(0, 0, getSelectorComponents($target));

        var uniqueContextPath = [];
        // Find the first element
        // repeat this step to construct a path of unique contexts
        var i = 0;
        var $context = $(document.body);
        while (path.length > 0 && i < path.length) {

            var $testQuery;
            // find the nearest parent that has a unique selector (within any previously determined context, otherwise within the DOM)
            for (i = 0; i < path.length; i++) {
                $testQuery = $(getSelectorFromComponents(path[i]), $context);
                if ($testQuery.length == 1) {
                    break;
                }
            }
            if (i < path.length) {
                $context = $testQuery;
                uniqueContextPath.push(path[i]);
                path.splice(i, path.length);
                // reset i to make sure we repeat the loop
                i = 0;
            }

        }

        var contextSelector = $(uniqueContextPath).map(function() { return getSelectorFromComponents(this); }).get().join(' ');
        console.log( contextSelector );
        console.log( $(contextSelector).length );

        var $message = $('#message');
        if ($message.length == 0) {
            $message = $('<div id="message" style="position:fixed; left: 0; right: 0; bottom: 0; background:rgba(0,0,0,.7); color: #fff; font-size:15px; text-align: center; padding:20px; font-family:Helvetica;"/>');
            $(document.body).append($message);
        }
        var message = 'Closest selector using only ID, class, tagName: <strong style="color:#f60;">'+contextSelector+'</strong>';
        message += '<br>100% match? <strong style="color:#f60;">'+(path.length == 0 ? 'YES' : 'NO')+'</strong>, Remaining DOM levels to narrow the match: <strong style="color:#f60;">'+path.length+'</strong>';
        $message.html(message);

        // Step 2.
        // Find a creative selector using :nth-child(). Other kinds of selectors could be included later.
        if (path.length > 0) {
            // TODO
        }

        // tag the element so we can find it later
        $target.addClass(selectedTag);
    });
});