How to Show an image preview before upload

HTML5 comes with File API spec, which allows you to create applications that let the user interact with files locally; That means you can load files and render them in the browser without actually having to upload the files. Part of the File API is the FileReader interface which lets web applications asynchronously read the contents of files .

Here’s a quick example that makes use of the FileReader class to read an image as DataURL and renders a thumbnail by setting the src attribute of an image tag to a data URL:

The html code:

<input type="file" id="files" />
<img id="image" />

The JavaScript code

document.getElementById("files").onchange = function () {
    var reader = new FileReader();

    reader.onload = function (e) {
        // get loaded data and render thumbnail.
        document.getElementById("image").src = e.target.result;
    };

    // read the image file as a data URL.
    reader.readAsDataURL(this.files[0]);
};