ensure.js

(function(window, jQuery) {

  /**
   *  ensure
   * 
   *  Only invokes callback if selector matches at least one node.
   * 
   *  @param string selector
   *  @param function callback
   *
   *  @dependencies jQuery
   * 
   *  @return bool|mixed
   */
  var ensure = window.ensure = function(selector, callback) {
    
    var targetNodes = jQuery(selector),
      targetNode = targetNodes.eq(0),
      numNodes = targetNodes.length;
    
    if(numNodes > 0 && typeof callback === 'function') {
      
      return callback.apply(this, [
        jQuery,
        targetNode,
        targetNodes
      ]);
      
    }
    
    return false;
    
  };

})(window, jQuery);

example.js

(function(window, $) {
  
  ensure('body.no-javascript', function($, target) {
    
    target.removeClass('no-javascript').addClass('has-javascript');
    
  });
  
  ensure('#node.that.doesnt-exist', function($, target) {
    
    // Won't fire
    
    target.data('answer', '42');
    
  });
  
})(window, jQuery);

Maybe change this to something like;

$('some.selector').exists(function() { /* conditional magic */ });