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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minutecanvas.toDataURL() turns the canvas’s current pixels into a data: URL, usually a PNG. To save a small image, assign that URL to a download link; to decode an existing data URL, separate its metadata header from its payload and convert the payload into a Blob. For larger exports, use canvas.toBlob() with URL.createObjectURL() to avoid building a large Base64 string in memory. A canvas containing cross-origin pixels without suitable CORS permission cannot be exported.
What canvas.toDataURL() returns
The method serializes the canvas bitmap as a data: URL. By default it requests PNG, and PNG is also the fallback if the browser does not support a requested format. The URL has a header that identifies the media type and, for Base64 data, the encoding; the encoded image follows the comma.
const dataUrl = canvas.toDataURL('image/png');
console.log(dataUrl); // data:image/png;base64,...You can request a lossy format such as JPEG and pass a quality value from 0 to 1. The quality parameter is relevant to lossy formats; it does not make PNG smaller.
const jpegUrl = canvas.toDataURL('image/jpeg', 0.85);For formats that support resolution metadata, the exported resolution is 96 dpi. This metadata does not change the bitmap’s pixel dimensions: a 1200-by-800 canvas still contains 1200 by 800 pixels.
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If the canvas has zero width or height, or exceeds the browser’s maximum canvas size, the result is the special string data:, rather than an image URL.
Download a canvas as an image
For a small export, create an anchor, set its download filename and use the data URL as its href. Run this in response to a user action, such as a button click.
function downloadCanvas(canvas, filename = 'canvas-image.png') {
const link = document.createElement('a');
link.download = filename;
link.href = canvas.toDataURL('image/png');
link.click();
}
document.querySelector('#download').addEventListener('click', () => {
downloadCanvas(document.querySelector('canvas'));
});For JPEG, use canvas.toDataURL('image/jpeg', 0.85) and give the download a .jpg filename. The requested MIME type and filename extension should agree. If you request an unsupported MIME type, the browser can fall back to PNG, so inspect the returned URL’s header when the actual format matters.
Decode a data URL into a Blob
A data URL is not just Base64 text: it begins with a metadata header such as data:image/png;base64,. When decoding one produced by FileReader.readAsDataURL(), or by canvas.toDataURL(), retain the MIME type from the header and decode only the payload after the first comma.
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function dataUrlToBlob(dataUrl) {
const comma = dataUrl.indexOf(',');
if (comma === -1) throw new Error('Invalid data URL: missing comma');
const header = dataUrl.slice(0, comma);
const payload = dataUrl.slice(comma + 1);
const mime = /^data:([^;,]+)/i.exec(header)?.[1] || 'application/octet-stream';
if (/;base64/i.test(header)) {
const binary = atob(payload);
const bytes = new Uint8Array(binary.length);
for (let i = 0; i < binary.length; i++) {
bytes[i] = binary.charCodeAt(i);
}
return new Blob([bytes], { type: mime });
}
return new Blob([new TextEncoder().encode(decodeURIComponent(payload))], {
type: mime
});
}
const dataUrl = canvas.toDataURL('image/png');
const blob = dataUrlToBlob(dataUrl);
const imageUrl = URL.createObjectURL(blob);The function handles both Base64 and percent-encoded data URL payloads. atob() returns a binary string, not a JavaScript array of bytes, so each character code is copied into a Uint8Array before creating the Blob. For large canvas exports, avoid this path when possible: toDataURL() first creates the full encoded string in memory.
Blob URLs refer to the in-memory Blob rather than embedding the image bytes in a string. Revoke a URL when it is no longer needed; if the user still needs to view or interact with the image, wait until that interaction is over.
const preview = document.querySelector('#preview');
preview.src = imageUrl;
preview.addEventListener('load', () => {
// Keep the URL while the preview is in use.
// Call URL.revokeObjectURL(imageUrl) when you replace or remove it.
}, { once: true });Use toBlob() for larger images
MDN generally recommends toBlob() with URL.createObjectURL() instead of toDataURL(). It avoids creating a large Base64 string, which can substantially increase memory use for large images. The callback can receive null, so handle that case before creating the URL.
function downloadCanvasBlob(canvas, filename = 'canvas-image.png') {
canvas.toBlob((blob) => {
if (!blob) {
console.error('The browser could not create the image Blob.');
return;
}
const url = URL.createObjectURL(blob);
const link = document.createElement('a');
link.download = filename;
link.href = url;
link.click();
// Revoke after the download has been initiated.
setTimeout(() => URL.revokeObjectURL(url), 0);
}, 'image/png');
}Pass the desired MIME type as the second argument, for example 'image/jpeg'; a third argument can supply the quality value for lossy output. Keep an object URL alive for as long as a preview or other consumer needs it, and revoke it after that use ends. For the simplest small image export, a data URL remains convenient; for larger output, the Blob route is usually the better fit.
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Why a canvas can be tainted
A canvas is origin-clean only when its pixels can legally be read by the page. Drawing an image or other pixel source from another origin without suitable CORS permission makes the canvas tainted. Calling toDataURL() or otherwise trying to read/export its pixels can then raise a SecurityError.
For an image you load yourself, set its CORS mode before assigning the source, and ensure the remote server sends an appropriate Access-Control-Allow-Origin response header. The remote server must permit your page’s origin; changing client-side code cannot override a server that does not grant access.
const image = new Image();
image.crossOrigin = 'anonymous'; // set before src
image.onload = () => {
const canvas = document.querySelector('canvas');
const context = canvas.getContext('2d');
canvas.width = image.naturalWidth;
canvas.height = image.naturalHeight;
context.drawImage(image, 0, 0);
// This works only if the image response permits CORS.
const dataUrl = canvas.toDataURL('image/png');
};
image.src = 'https://example.com/image.png';Using a server-side proxy may be an option only if you control or are authorized to use that service and it fetches the resource safely. Do not treat a proxy as a way to bypass another site’s access controls.
Common errors and fixes
- The result is
data:,. Check that both canvas dimensions are nonzero and within the browser’s maximum canvas size. Reduce the dimensions or export a smaller region if necessary. SecurityErrorappears during export. The canvas may contain cross-origin pixels. Load the source with CORS enabled and confirm the image server permits your origin, or use content you are entitled to read.- The downloaded file is empty or corrupt. Confirm drawing completed before export, verify the canvas dimensions, and check the returned URL begins with an expected image type such as
data:image/png;base64,. - The decoded bytes are invalid. Do not pass the complete data URL to
atob(); split at the first comma and decode only the payload when the header specifies Base64. For a percent-encoded payload, decode it withdecodeURIComponent()instead. - The file type does not match the requested type. A browser may not support the requested MIME type and can fall back to PNG. Check the data URL or Blob type and use an extension that matches the actual output.
- The image export uses too much memory. Replace
toDataURL()withtoBlob()and an object URL, and revoke the URL when its consumer is finished. - The download does not start from a delayed callback. Trigger the operation from a clear user action, such as a button click. Browser download handling can restrict downloads that are not associated with user interaction.
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Choose the right export path
| Need | Use | Why |
|---|---|---|
| Quickly display or download a small canvas image | toDataURL() |
Returns a directly usable image URL, with no separate Blob URL lifecycle. |
| Save or preview a large image | toBlob() and URL.createObjectURL() |
Avoids holding the whole Base64-encoded image in a string. |
| Convert an existing data URL to binary data | Parse header and payload, then create a Blob | Preserves the MIME type and allows Blob-based downloads or previews. |
| Export pixels drawn from another origin | Make the source CORS-permitted before drawing | A tainted canvas cannot be read or exported by client-side code. |
Frequently Asked Questions
Does canvas.toDataURL() return only Base64?
No. It returns a complete data: URL, including a media-type and encoding header before the comma-separated payload.
Can toDataURL() export a canvas as JPEG?
Yes. Request image/jpeg and optionally provide a quality value from 0 to 1; the browser can fall back to PNG if the requested type is unsupported.
Does converting a data URL fix a tainted canvas?
No. Converting an already available data URL to a Blob does not grant permission to read cross-origin canvas pixels. The source must be CORS-permitted before drawing.
Quick Recap
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