Question

I want to generate some <select elements dynamically. I’m copying an existing one and only change the name and the onchange-Event. This is where my problem is: Every <select> has its own ID. When the onChange-Event fires it should respond the Value and the ID of the Element. I don’t know how to define the JavaScript Closure exactly. I tried it with “this.value”, but apparently this does not work…
I already found this example, but it did not work for me at all :/

The JavaScript:

var i = 0;
var selectArray = [];

function addSelector(){

var container = document.getElementById("check0");

selectArray[i] = document.getElementsByName("select0")[0].cloneNode(true);
selectArray[i].name = 'select'+i;
selectArray[i].onchange = function(v, i) {
        return function() {
           changeType(v, i)
        }
    }(this.value, i);

container.appendChild(selectArray[i]);
}

function changeType(selected, i) {
    switch (selected) {
        case 'One':
           alert(selected+' , '+i);
            break;
        case 'Two':
            alert(selected+' , '+i);
            break;          
        case 'Three':
            alert(selected+' , '+i);
            break;
        case 'Four':
            alert(selected+' , '+i);
            break;
        case 'Five':
            alert(selected+' , '+i);
            break;
    }
}

(I tried to post the HTML aswell, but for some reason i wasnt able to do this 😉 )

Here is the fiddle

Answer 1

this.value isn’t known here:

function addSelector(){
    /* ... */
    selectArray[i].onchange = (function(v, i) {
        return function() {
            changeType(v, i)
        }
    })(this.value, i);
    /* ... */
}

this will be undefined (in strict mode) or window (normal mode). How should JavaScript know that this is a reference to the created/cloned <select> object? this will be correct inside of your onchange, so you don’t have to include this by closure, only the index i has to be included:

selectArray[i].onchange = (function(index) {
    return function() {
        changeType(this.value, index)
    }
})(i);

Notice that your syntax for alert is wrong – you should use alert(selected + " " + i) instead, as the second argument will be ignored.

See also:

• JSFiddle Demo
• MDN: this