Load data from google sheet on start up - google-apps-script

Question

I tried to retrieve a list of agents who are stored in a google sheet. The problem that I am facing right now is that I can get the list of agents in the script file. However, when I try to pass it to an html file, I keep getting null for the list. I absolutely has no idea how that can be.
Can someone give me an idea where I should look for?
My script:
var DB_URL = "";
var AGENT_DB = "";
var CREATED_ON_IDX = 0;
var NAME_IDX = 1;
var EMAIL_IDX = 2;
function agentService_getAgents() {
var ss = SpreadsheetApp.openByUrl(DB_URL);
var sheet = ss.getSheetByName(AGENT_DB);
var dataRange = sheet.getDataRange();
var agents = [];
var values = dataRange.getValues();
for (var i = 1; i < values.length; ++i) {
var row = values[i];
var name = row[NAME_IDX];
var email = row[EMAIL_IDX];
var createdOn = row[CREATED_ON_IDX];
var agent = new Agent(name, email, createdOn);
agents[i-1] = agent;
}
Logger.log(agents);
return agents;
}
Ajax call in Html
<script type="text/javascript">
function onSuccess(agents) {
var $table = $("table");
console.log(agents);
}
google.script.run.withSuccessHandler(onSuccess)
.agentService_getAgents();
</script>
So Logger.log(agents) gives me a list of agent; but console.log(agents) gives me null.