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Execute Function After Canvas toDataURL() Function is Complete

I'm converting images to base64 using canvas. What i need to do is convert those images and then show the result to the user (original image and base64 version). Everything works as expected with small images, but when i try to convert large images (>3MB) and the conversion time increases, the base64 version is empty.
This might be is caused because the result is shown before the toDataURL() function is completed.
I need to show the result after all the needed processing has ended, for testing purposes.
Here's my code:
var convertToBase64 = function(url, callback)
{
var image = new Image();
image.onload = function ()
{
//create canvas and draw image...
var imageData = canvas.toDataURL('image/png');
callback(imageData);
};
image.src = url;
};
convertToBase64('img/circle.png', function(imageData)
{
window.open(imageData);
});
Even though i'm using image.onload() with a callback, i'm unable to show the result after the toDataURL() has been processed.
What am i doing wrong?
UPDATE: I tried both the solutions below and they didn't work. I'm using AngularJS and Electron in this project. Any way i can force the code to be synchronous? Or maybe some solution using Promises?
UPDATE #2: #Kaiido pointed out that toDataURL() is in fact synchronous and this issue is more likely due to maximum URI length. Since i'm using Electron and the image preview was for testing purposes only, i'm going to save the file in a folder and analise it from there.

Your code seems absolutely fine. Not sure why isn't working you. Maybe, there are some issues with your browser. Perhaps try using a different one. Also you could use a custom event, which gets triggered when the image conversion is competed.
// using jQuery for custom event
function convertToBase64(url) {
var image = new Image();
image.src = url;
image.onload = function() {
var canvas = document.createElement('canvas');
canvas.width = image.width;
canvas.height = image.height;
var ctx = canvas.getContext('2d');
ctx.drawImage(image, 0, 0);
var imageData = canvas.toDataURL();
$(document).trigger('conversionCompleted', imageData);
};
};
convertToBase64('4mb.jpg');
$(document).on('conversionCompleted', function(e, d) {
window.open(d);
});

This approach might work for you. It shows the image onscreen using the native html element, then draws it to a canvas, then converts the canvas to Base64, then clears the canvas and draws the converted image onto the canvas. You can then scroll between the top image (original) and the bottom image (converted). I tried it on large images and it takes a second or two for the second image to draw but it seems to work...
Html is here:
<img id="imageID">
<canvas id="myCanvas" style="width:400;height:400;">
</canvas>
Script is here:
var ctx;
function convertToBase64(url, callback)
{
var image = document.getElementById("imageID");
image.onload = function() {
var canvas = document.getElementById("myCanvas");
canvas.width = image.naturalWidth;
canvas.height = image.naturalHeight;
ctx = canvas.getContext("2d");
ctx.drawImage(image,0,0);
var imageData = canvas.toDataURL('image/png');
ctx.fillStyle ="#FFFFFF";
ctx.fillRect(0,0,canvas.width,canvas.height);
callback(imageData);
};
image.src = url;
};
var imagename = 'images/bigfiletest.jpg';
window.onload = function () {
convertToBase64(imagename, function(imageData) {
var myImage = new Image();
myImage.src = imageData;
ctx.drawImage(myImage,0,0);
});
}
Note that I also tried it without the callback and it worked fine as well...

Create base64 encoded images with JavaScript

Since images are data, we can write our code as
<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA
AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO
9TXL0Y4OHwAAAABJRU5ErkJggg==" alt="Red dot" />
Now my point is: can we create that base64 data with javascript? Is there any framework for it?
My actual requirement is that I have a string like "Cow" and I want it as an image.
Note: I do not want a server call for it. I know I can call server side code by passing "Cow" as a parameter. My server side code can generate that image, but I want do this from the browser with JavaScript.

You can create an image with canvas. There are many canvas frameworks which can help you. Then you can "export" your canvas to base64 string.
Try this sample with jCanvas:
HTML
<p>
Result:
</p>
<p>
<textarea id="result" cols="60" rows="10"></textarea>
</p>
<p>
<input id="click" type="button" value="Convert to base64">
</p>
<p>
Canvas<br><canvas id="myCanvas" width="100" height="100"/>
</p>
JavaScript
$(function(){
var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");
ctx.fillStyle = "#FF0000";
ctx.fillRect(0, 0, 80, 80);
$('#click').click(function() {
$('#result').html($("canvas").getCanvasImage());
});
});
Demo

I hope I got your question right. You want the actual data and not the uri??
I found this small snippet from MuniR, I think it is what you wanted.
The thing is we cannot seem to get away from the canvas, create it with the size of the image, paint the image and use the canvas object to store it!!-Not necessarily USE it... Hope it helps, and good luck
function getBase64FromImageUrl(url) {
var img = new Image();
img.setAttribute('crossOrigin', 'anonymous');
img.onload = function () {
var canvas = document.createElement("canvas");
canvas.width =this.width;
canvas.height =this.height;
var ctx = canvas.getContext("2d");
ctx.drawImage(this, 0, 0);
var dataURL = canvas.toDataURL("image/png");
alert(dataURL.replace(/^data:image\/(png|jpg);base64,/, ""));
};
img.src = url;
}

var element = new Image();
element.src = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==";
If you wants to add id to your body:
document.body.appendChild(element);
You have to use appendChild on the parent element of where you want to display the image. It will be automatically loaded.

getting html canvas image as javascript object

I want to get html canvas image as javascript object. I looked samples from internet. But all samples are getting images from a source to canvas like this:
<body>
<canvas id="myCanvas" width="578" height="400"></canvas>
<script>
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
var imageObj = new Image();
imageObj.onload = function() {
context.drawImage(imageObj, 69, 50);
};
imageObj.src = 'http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg';
</script>
</body>
But my problem: There is an image on canvas element that created by pencil. I want to get that image as object and post to server. But I could not get.

have you tried canvas.toDataURL()?
more hardcore way might be canvas.getContext('2d').getImageData() would return ImageData object.

canvas.toblob fails when a patterned fill is used

I've been trying to find a method to save the canvas to file. My image is too large to use dataToUrl, so I have been trying various toblob methods. It seems that when a patterned fill is used, toblob does not work. Would anyone be able to to me if it is possible for this to work or if there is another way to accomplish this? Thanks
jfiddle example
<!DOCTYPE html>
<html>
<canvas id="myCanvas" width="300" height="150" style="border:1px solid #d3d3d3;">Your browser does not support the HTML5 canvas tag.</canvas>
<script src="jquery.min.js"></script>
<script src="canvas-to-blob.js"/></script>
<script src="FileSaver.js-master\FileSaver.js"></script>
<script>
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.clearRect(0, 0, c.width, c.height);
var img = new Image();
img.src = "http://www.w3schools.com/tags/img_lamp.jpg";
var pat = ctx.createPattern(img, "repeat");
ctx.rect(0, 0, 150, 100);
//Works with color, but not with pattern
ctx.fillStyle = pat;
//ctx.fillStyle = 'blue';
ctx.fill();
try {
var isFileSaverSupported = !! new Blob();
} catch (e) {
alert(e);
}
alert("toBlob");
c.toBlob(function (blob) {
alert("success");
saveAs(blob, "TruVue.png");
});
</script>
</html>

The reason is due to CORS, images from other domains are restricted - you can show them on the canvas but you cannot extract their bitmap.
As toBlob is a bitmap extracting method like toDataURL or getImageData you won't be able to use these images.
There are a couple of work-arounds:
Upload the image to your own server and load it from there (same domain as you use for the page).
Modify the other server to include Access-Control-Allow-Origin headers (in this case it will probably not be do-able).
Use your own server as an image proxy
BTW: You should also use the image's onload event to be sure the image gets proper loaded before using the image:
var img = new Image();
img.onload = drawFunction;
img.src = "http://www.w3schools.com/tags/img_lamp.jpg";
function drawFunction() {
/// draw here
}

context.drawImage behaving weirdly

I have:
<canvas id='canvas' width="300" height="409" style="border:2px solid darkblue" >
</canvas>
And then:
<script type="text/javascript">
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
var image = new Image();
image.src = 'http://4.bp.blogspot.com/...-21+Kingfisher.JPG';
alert(image.src);
context.drawImage(image, 0, 0, 300, 400);
</script>
In IE 10, the image is painted as "to be expected". However, when I remove that alert statement, the picture is not painted!
In Chrome, no image is painted on my local PC, whether with or without the alert statement.
What could be happening? The fiddle is here

That is because loading images is an asynchronous operation. The alert call helps the browser to wait a bit so the image loading can finish. Therefor the image will be available at drawImage that follows.
The correct way to implement this is to use the code this way:
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
var image = new Image(); //document.createElement('img'); for Chrome due to issue
// add a onload handler that gets called when image is ready
image.onload = function () {
context.drawImage(this, 0, 0, 300, 400);
}
// set source last so onload gets properly initialized
image.src = 'http://4.bp.blogspot.com/...-21+Kingfisher.JPG';
The draw operation inside the callback for onload could just as easily have been a function call:
image.onload = nextStep;
// ...
function nextStep() {
/// draw image and other things...
}