I loaded my local .json file with fetch, and I registered an event handler on a button element.

Here is my code:

        let tagName;
        function start(){
            fetch("./product.json")
            .then(response=>response.json())
            .then(data=>addItem(data,tagName))
            .catch(error=>console.log(error));            
        }
        start();
        
        function addItem(data,tagName){                
            var container = document.getElementById("itemBox");
            for(let i=0;i<data.length;i++){                       
                var div = document.createElement("div");
                var img = data[i].itemImg;                                  
                div.className = "imgBox";
                div.innerHTML = "<img src= "+img+">";
                container.appendChild(div);
            }
        };
        
        let tagBtn = document.getElementById("tagsubmit");
        tagBtn.onclick = function(){
            document.getElementById("tmp").innerHTML = "HELPME";
        }

When I opened this html file, json file is loaded&rendered succesfully, but the button with event handler doesn't work.

How can I make both of them work successfully?