applying an event handler to newly created objects

Question

So my goal is to have 5 boxes and every time one box is clicked a new box appears. The code I wrote for that is this:

window.onload = function(){
    var boxLis         


        

Answer 1

If you create boxes dynamically after the window.onload handler has already run, then you will have to run some additional code on those dynamically created boxes that assigns the click handler to them after they have been created.

function clickHandler(eo){
    if(eo.target.style.backgroundColor != "black") {
        eo.target.style.backgroundColor = "black";
        var box = document.createElement("div");
        box.setAttribute("class","box");

        // add this line of code to assign the click handler
        box.onclick = clickHandler;

        box.setAttribute("id",document.getElementsByClassName("box").length++);
        document.getElementById("Master").appendChild(box);
    }
    else eo.target.style.backgroundColor = "white";
}

---

Or, you can use delegated event handling and handle the events from a common parent that is not dynamically created.

Delegated event handling uses "event bubbling" where events bubble up their parent chain so you can attach a click handler to a common parent and then check e.target in that click handler to see if the click occurred on one of your box elements and then process it one place. In cases of dynamically added content, this can work very well.

Delegated event handling in your code would look something like this:

window.onload = function(){
    // put click handler on common box parent and use event bubbling
    document.getElementById("Master").addEventListener("click", clickHandler);
}

function clickHandler(eo){
    // if this click occurred on one of my boxes
    if (hasClass(eo.target, "box"))
        if(eo.target.style.backgroundColor != "black") {
            eo.target.style.backgroundColor = "black";
            var box = document.createElement("div");
            box.setAttribute("class","box");
            box.setAttribute("id",document.getElementsByClassName("box").length++);
            document.getElementById("Master").appendChild(box);
        }
        else eo.target.style.backgroundColor = "white";
    }
}

// utility function for checking a class name
// could also use .classList with a polyfill for older browsers
function hasClass(elem, cls) {
    var str = " " + elem.className + " ";
    var testCls = " " + cls + " ";
    return(str.indexOf(testCls) !== -1) ;
}