getRowCount() is for server side , what if I want through client side?

Use Glideaggregate with GlideAjax to get Count

Client Script
var ga = new GlideAjax('MyCustomAjax');
ga.addParam('sysparm_name', 'helloWorld');
ga.addParam('sysparm_hoge', g_form.getValue("email")); // When you want to pass a value
ga.getXMLAnswer(callback);
 function callback(answer){
  var getanswer = answer.evalJSON();
    g_form.addInfoMessage(getanswer.var1); //Display "Hello"
    g_form.addInfoMessage(getanswer.var2);
}
  Script Include
var MyCustomAjax = Class.create();
MyCustomAjax.prototype = Object.extendsObject(AbstractAjaxProcessor, {
     helloWorld: function() {
      var count = new GlideAggregate('incident');
count.addAggregate('COUNT');
count.query();
var incidents = 0;
if (count.next()) {
   return count.getAggregate('COUNT');
 }
           },
     type: 'MyCustomAjax'
});