<apex:page StandardController="Product__c" extensions="ExistingController"  >
   <apex:form id="fm" >
   <apex:actionFunction name="invoke" action="{!search}" reRender="pb">
<apex:param name="firstParam" assignTo="{!state}" value="" />
</apex:actionFunction>
       <apex:pageBlock id="pb" title="Products" mode="edit" >
           <apex:pageBlockSection id="pbs" title="Search/Create a Product record" columns="2">
               <apex:inputField id="ifname" value="{!Product__c.name}"  />
               <apex:inputField value="{!Product__c.Description__c}" />
               <apex:inputField value="{!Product__c.Total_Inventory__c}"  />
               <apex:inputField value="{!Product__c.Price__c}" />
               <apex:inputField value="{!Product__c.Stock_Quantity__c}" />
           </apex:pageBlockSection>
               <apex:pageBlockButtons >
               <apex:commandButton value="reset"  action="{!Reset}"/>
               <apex:commandButton value="Save" action="{!Save}" rendered="{!flag}"/>
               <apex:commandButton value="Search" OnClick="return nameNull()"/>
           </apex:pageBlockButtons>
          
           <apex:pageBlockTable id="pbt" var="pb" value="{!prodRecordList}">
               <apex:column value="{!pb.Name}" />
               <apex:column value="{!pb.Description__c}"/>
               <apex:column value="{!pb.Total_Inventory__c}"/>
               <apex:column value="{!pb.Price__c}"/>
               <apex:column value="{!pb.Stock_Quantity__c}"/>
           </apex:pageBlockTable>
           </apex:pageBlock>
      
      
   </apex:form>
   <script>
       function nameNull(){
           var text= document.getElementById("{!$Component.fm.pb.pbs.ifname}").value;
           if(text == '')
           {alert('Please enter value in Product Name field');
           }
           else
                {           
        invoke(text);
                                   }
        return false;
        
                                           }
   </script>
</apex:page>

 

 

Controller Code:

 

public class ExistingController {

   public string nameStr{ get; private set; }
   Public List<product__c> prodRecordList {get; set;}
   Public product__c prod {get; set;}
   public boolean flag {get; set;}
   public string state{get;set;}
 
   public ExistingController(ApexPages.StandardController stdController) {
       this.prod= (product__c)stdController.getRecord();
   }
 
   public pageReference search () {

       prodRecordList =  [SELECT id, Name, Description__c, Total_Inventory__c, Price__c, Stock_Quantity__c from product__c where name=:state];
       System.debug('@@prodRecordList'+prodRecordList);
       if(prodRecordList.size()==0 )
      
           flag = true;
           return null;
   }
   public pageReference Reset() {

       PageReference newpage = new PageReference(System.currentPageReference().getURL());
       newpage.setRedirect(true);
       return newpage;  
          }
      
  public pageReference Save()  {
  INSERT prod;
  pageReference newpage = new PageReference('/apex/recordsaved');
  return newpage;
  }
}



Correction1:You need to put "return" keyword when calling jscript code in commandbutton. otherwise use HTML input button

Correction 2: Use apex:param in action function for passing parameter

Please let me know if you have any doubts