[Solved] ActionFunction reloads the page even I have a reRender and returns null

Lance Shi Asks: ActionFunction reloads the page even I have a reRender and returns null
My <apex:actionFunction> keeps reloading the VF page even though it does have a reRender attribute and the controller method returns null. Below are part of my code:


<apex:pageMessages id="page_message" />
<apex:actionFunction name="testConnection"  action="{!testConnection}" reRender="page_message" onComplete="displayConnectionInfo();" />

onComplete function:

function displayConnectionInfo() {
    return false;

Controller method:

public pageReference testConnection() 
    ApexPages.Message connection_info = Util.testConnection();

    return null;

My another page has a similar button and actionfunction and does work very well without reloading the page. There must be something I missed here but I am not sure what is it.

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