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resizing an image on the front-end before sending to a server
// Using this code, we can retrieve an image from a user's filesystem, resize the image, and then upload the image
// to a server using AJAX. Because we use base64 encoding, we can just include the image data as just another string value
// in a JSON payload.
// So we can use AJAX to send the file to a server, which is convenient.
// We have one line of relevant html
// get file in the first place => <input type="file" custom-on-change="onAcqImageFileChange" class="form-control">
$scope.onAcqImageFileChange = function (e) {
var file =[0];
$scope.acqImageFile = file; // store reference to file
function convertToBase64(file, cb) {
var reader = new FileReader();
reader.onload = function (e) {
reader.onerror = function (e) {
function resizeImage(base64Str) {
var img = new Image();
img.src = base64Str;
var canvas = document.createElement('canvas');
var MAX_WIDTH = 400;
var MAX_HEIGHT = 350;
var width = img.width;
var height = img.height;
if (width > height) {
if (width > MAX_WIDTH) {
height *= MAX_WIDTH / width;
width = MAX_WIDTH;
} else {
if (height > MAX_HEIGHT) {
width *= MAX_HEIGHT / height;
height = MAX_HEIGHT;
canvas.width = width;
canvas.height = height;
var ctx = canvas.getContext('2d');
ctx.drawImage(img, 0, 0, width, height);
return canvas.toDataURL();
$scope.submitImageSelection = function () {
// when the user finally selects the image they want, and clicks submit, we run this fn
var imgFile = $scope.acqImageFile;
convertToBase64(imgFile, function (err, data) {
if (err) {
/// handle error
// resize the image like a boss
data = resizeImage(data);
// finally we can send the data to a server as
// just another field in a JSON payload
SomeService.sendDataToServer(data, function(err){
// voila
// note that to display the image selection back to the user before they choose their final selection, you may want to convert to
// base64 each time they change selections.
// to display the image, you can use ng-src:
// <img class="overlay-image" ng-src="{{acqImageData ? acqImageData : '/assets/img/placeholder.png'}}">
// <the end>

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commented Aug 14, 2019

It works fine if you add an "onload" listener on "img" in "resizeImage(base64Str)" function and move the rest part of function inside the onload function.

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