<Join Type='LEFT' ListAlias='customer'>
   <Eq>
     <FieldRef Name='CustomerName' RefType='Id'/>
     <FieldRef List='customer' Name='ID'/>
   </Eq>
</Join>

var query = SPHelper.query().from("Orders)
              .join({on: "CustomerName", as: "customer"})
              .select(["OrderId", "customer.Address as CustomerAddress"]);