First Expression in a Javascript For Loop - what's with the comma?

Question

I'm working through an HTML5 drag and drop example http://www.sitepoint.com/html5-file-drag-and-drop/, but can't figure out exactly what is happening in the loop at the end of this function-

function FileSelectHandler(e) {

    // cancel event and hover styling
    FileDragHover(e);

    // fetch FileList object
    var files = e.target.files || e.dataTransfer.files;

    // process all File objects
    for (var i = 0, f; f = files[i]; i++) {
        ParseFile(f);
    }

}

As far as I know, the first expression should just be i=0. What is happening with the , f? As long as the files array contains key i, i gets incremented, f is parsed and it then looks for files[i] again, right?

Answer 1

In a for loop, you can have multiple initializers separated by commas. That's what you have there, combined with a lazy (and misleading) var. In this particular case, it's equivalent to:

function FileSelectHandler(e) {
    var i, f, files;

    // cancel event and hover styling
    FileDragHover(e);

    // fetch FileList object
    files = e.target.files || e.dataTransfer.files;

    // process all File objects
    for (i = 0; f = files[i]; i++) {
        ParseFile(f);
    }
}

...because var is badly misunderstood in JavaScript. But probably a better example of using multiple initializers would be:

var a = [1, 2, 3], index, len;

for (index = 0, len = a.length; index < len; ++index) {
    // Do something with a[index]
}

There, with the misleading var removed, we can see that there are two distinct initializers at the beginning of the for statement.