javascript - Loading an image to a <img> from <input file>

Question

I'm trying to load an image selected by the user through an element.

I added a onchange event handler to the input element like this:

<input type="file" name="picField" id="picField" size="24" onchange="preview_2(this);" alt=""/>

and the preview_2 function is:

var outImage ="imagenFondo";
function preview_2(what){
    globalPic = new Image();
    globalPic.onload = function() {
        document.getElementById(outImage).src = globalPic.src;
    }
    globalPic.src=what.value;
}

where outImage has the id value of the tag where I want the new picture to be loaded.

However, it appears that the onload never happens and it does not load anything to the html.

What should I do?

Answer 1

In browsers supporting the File API, you can use the FileReader constructor to read files once they have been selected by the user.

Example

document.getElementById('picField').onchange = function (evt) {
    var tgt = evt.target || window.event.srcElement,
        files = tgt.files;

    // FileReader support
    if (FileReader && files && files.length) {
        var fr = new FileReader();
        fr.onload = function () {
            document.getElementById(outImage).src = fr.result;
        }
        fr.readAsDataURL(files[0]);
    }

    // Not supported
    else {
        // fallback -- perhaps submit the input to an iframe and temporarily store
        // them on the server until the user's session ends.
    }
}

Browser support

• IE 10
• Safari 6.0.2
• Chrome 7
• Firefox 3.6
• Opera 12.02

Where the File API is unsupported, you cannot (in most security conscious browsers) get the full path of a file from a file input box, nor can you access the data. The only viable solution would be to submit the form to a hidden iframe and have the file pre-uploaded to the server. Then, when that request completes you could set the src of the image to the location of the uploaded file.