Drag out files from browser using javascript

Question

I'm trying to modify some code for dragging out files from your browser window I found here: http://www.thecssninja.com/javascript/gmail-dragout

In my code I want to use for loops so I can handle a large number of files without repeating code over and over.

This is what I have:

<a href = "file0" id = "0" draggable = "true" data-downloadurl="application/pdf:file0name.pdf:file:///C:/filepath/file0.pdf">file0</a>
<a href = "file1" id = "1" draggable = "true" data-downloadurl="application/pdf:file1name.pdf:file:///C:/filepath/file1.pdf">file1</a>
<a href = "file2" id = "2" draggable = "true" data-downloadurl="application/pdf:file2name.pdf:file:///C:/filepath/file2name.pdf">file2</a>
<a href = "file3" id = "3" draggable = "true" data-downloadurl="application/pdf:file3name.pdf:file:///C:/filepath/file3name.pdf">file3</a>

<script type = "text/javascript>
    var file_count = 3;
var fileDetails = [];

var files = new Array(file_count);
for(var i=0; i<=file_count; i++) {
    files[i] = document.getElementById(i);
}

if(typeof files[0].dataset === "undefined") {
    for (i=0; i<=file_count; i++) {
        fileDetails[i] = files[i].getAttribute("data-downloadurl");
    }
}else {
    for (i=0; i<=file_count; i++) {
        fileDetails[i] = files[i].dataset.downloadurl;
    }
}

//I'm pretty sure the problem starts here.
//If you substitue i for a specific element from the array and comment out the for loop, the script works just fine for the element specified.
for(i=0; i<=file_count; i++) {
    files[i].addEventListener("dragstart",function(evt){
        evt.dataTransfer.setData("DownloadURL",fileDetails[i]);
    },false);
}

I'm fairly certain the problem starts where I have it labeled, I'm not sure why there is a problem or how to solve it.

Some things I should point out:

This only works in Chrome. This is not an issue for me.

I want this to handle 20+ files.

Kinlan · Accepted Answer · 2013-06-04 15:16:42Z

The problem does indeed start where you think it does

//I'm pretty sure the problem starts here.
//If you substitue i for a specific element from the array and comment out the for loop, the script works just fine for the element specified.
for(i=0; i<=file_count; i++) {
    files[i].addEventListener("dragstart",function(evt){
        evt.dataTransfer.setData("DownloadURL",fileDetails[i]);
    },false);
}

Basically your dragstart event listener is bound to i, which at the time of execution is the last element in the list.

The following will work better.

//I'm pretty sure the problem starts here.
//If you substitue i for a specific element from the array and comment out the for loop, the script works just fine for the element specified.
for(i=0; i<=file_count; i++) {
    files[i].addEventListener("dragstart", (function(idx) {
        return function(evt){
           evt.dataTransfer.setData("DownloadURL",fileDetails[idx]);
        }
    })(i),false);
}

In the above code you will be creating and returning an object of type function with the correct value of i bound - essentially the closure is created at a different level and thus when accessed will get the correct data.

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