File retrieval and changing img element with Bootstrap

Sharing a smaller code snippet / mini tutorial / example since I have found it very useful in connection to HTML preview display of cam image or an uploaded image, depending on what you want really 🙂
So, the example is using Twitters Bootstrap, jQuery and beloved base64. I know the last one is just an encoding, but I like it nonetheless!

In the following code a simple example is provided with an image which will be replaced later with uploaded image without reloading.

<!DOCTYPE html>
<html lang="en">
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">

<title>demo canvas example</title>

<!-- Bootstrap Definitions -->
<link href="css/bootstrap.min.css" rel="stylesheet">
<!-- Choosen font -->
<link href='' rel='stylesheet' type='text/css'>


<div class="row text-center" id="pass" style="padding-bottom: 20px;">
<div class="col-sm-10"><h2>Image placeholder</h2></div>
<div class="col-sm-12 ">
<img id="imagePreview" height="220" width="355" src="chosenImage.jpg"
class="img-responsive center-block" alt="">
<!-- regarding btn-file class check at the bottom for a neat solution of making design same as for other Bootstrap buttons -->
<span class="btn btn-primary btn-file">
<!-- File upload button and file chooser -->
Upload <input type="file" id="fileuploadform">

<!-- Scripts loading -->
<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
<script src=""></script>
<!-- Include all compiled plugins (below), or include individual files as needed -->
<script src="js/bootstrap.min.js"></script>

<!-- This script can be seen in the next code section -->
.... JS script part

Javascript nicely separated 🙂

var trimmedReaderResult;

//If upload form is used, the following function will get triggered
$("#fileuploadform").on('change', function () {

//following the workflow
function previewAndSendDoc(whichDiv) {
if (whichDiv === 1) {
var uploadformid = 'fileuploadform';
//Getting image named imagePreview, this <img> element will be used for manipulation
var previewImg = document.getElementById('imagePreview');
//getting the file from the file upload button
var uploadform = document.getElementById(uploadformid).files[0];
//Initializing FileReader
var reader = new FileReader();

In this function reader function callback will be called after it finishes the file and replace image of previewImage element newly received file. After that in this case the base64 of the image will be taken and truncated until only binary data without prefix remains for further manipulation

reader.onloadend = function () {
previewImg.src = reader.result;
trimmedReaderResult = reader.result.substring(reader.result.indexOf(',')+1);
//getting pure base64 of image

Function which will go to the above function after readAsDataUrl is finished

if (uploadform) {
} else {
  previewImg.src = "";

And addition for colouring button for a file upload to have the same design as other Boostrap buttons:

.btn-file {
    position: relative;
    overflow: hidden;
.btn-file input[type=file] {
    position: absolute;
    top: 0;
    right: 0;
    min-width: 100%;
    min-height: 100%;
    font-size: 100px;
    text-align: right;
    filter: alpha(opacity=0);
    opacity: 0;
    outline: none;
    background: white;
    cursor: inherit;
    display: block;
Matjaz Trcek
Matjaz Trcek
SRE @ Magnolia CMS

Working as an SRE in Magnolia CMS. In my free time I work on many side projects some of which are covered in this blog.