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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallcanvas.toDataURL() turns the current HTML canvas bitmap into a data: URL. To save a small PNG or JPEG, assign that URL to an anchor’s href and trigger a download. To decode the string for upload or further processing, split it at the first comma, read the MIME type from the header, and Base64-decode the payload. For large canvases, use canvas.toBlob() and URL.createObjectURL() instead: it avoids keeping the entire encoded image in one JavaScript string.
What canvas.toDataURL() returns
Calling canvas.toDataURL() serializes the canvas’s current pixels as a data: URL:
const dataUrl = canvas.toDataURL('image/png');
console.log(dataUrl); // data:image/png;base64,...The default format is PNG. You may request another format, such as JPEG, by passing a supported MIME type. For lossy formats, the optional quality argument is a number from 0 to 1; for example, 0.85 requests approximately 85-percent quality. If the browser does not support the requested type, it falls back to PNG, so inspect the returned prefix when the exact format matters. Formats that support resolution metadata are exported at 96 dpi.
Draw first, export second
The method captures the bitmap at the moment it is called. A minimal complete example is:
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<canvas id="art" width="800" height="450"></canvas>
<script>
const canvas = document.querySelector('#art');
const ctx = canvas.getContext('2d');
ctx.fillStyle = '#1769aa';
ctx.fillRect(0, 0, canvas.width, canvas.height);
ctx.fillStyle = 'white';
ctx.font = '32px sans-serif';
ctx.fillText('Saved canvas', 40, 80);
const pngDataUrl = canvas.toDataURL('image/png');
console.log(pngDataUrl.slice(0, 30));
</script>Canvas dimensions are bitmap dimensions, not merely CSS display dimensions. Set the width and height attributes (or the corresponding properties) before drawing if you need a particular output size.
Download a canvas as PNG or JPEG
For a small export, create an anchor, set its download filename and use the data URL as href:
function downloadCanvas(canvas, type = 'image/png', quality) {
const extension = type === 'image/jpeg' ? 'jpg' :
type === 'image/webp' ? 'webp' : 'png';
const dataUrl = canvas.toDataURL(type, quality);
const link = document.createElement('a');
link.download = `canvas-image.${extension}`;
link.href = dataUrl;
link.click();
}
downloadCanvas(document.querySelector('#art'), 'image/png');
downloadCanvas(document.querySelector('#art'), 'image/jpeg', 0.85);The browser may apply its own download or security policy, particularly when the call is not made as a result of a user gesture. Invoke the function from a click handler for the most consistent behavior:
document.querySelector('#save').addEventListener('click', () => {
downloadCanvas(document.querySelector('#art'), 'image/png');
});PNG preserves transparency and is a good default for drawings, text and screenshots. JPEG is smaller for photographic content but has no alpha channel; transparent pixels are typically composited against a background. WebP can be requested where supported, but always check the returned MIME prefix if your application depends on it.
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Decode a data URL into a Blob
A data URL has a metadata header, a comma, and the encoded data. A Base64 example looks like data:image/png;base64,iVBORw0KGgo.... Do not pass the complete string directly to atob(); remove the header first. The following function handles both Base64 and percent-encoded data URLs:
function dataUrlToBlob(dataUrl) {
const [header, payload] = dataUrl.split(',', 2);
if (!header || payload === undefined || !header.startsWith('data:')) {
throw new Error('Invalid data URL');
}
const mime = /data:([^;]+)/.exec(header)?.[1] ||
'application/octet-stream';
if (header.includes(';base64')) {
const binary = atob(payload);
const bytes = Uint8Array.from(binary, character => character.charCodeAt(0));
return new Blob([bytes], { type: mime });
}
const bytes = new TextEncoder().encode(decodeURIComponent(payload));
return new Blob([bytes], { type: mime });
}
const dataUrl = canvas.toDataURL('image/png');
const blob = dataUrlToBlob(dataUrl);
console.log(blob.type, blob.size);Blob.type reports the MIME type from the header, while Blob.size is the decoded byte count. You can upload the Blob with fetch or display it through an object URL:
const imageUrl = URL.createObjectURL(blob);
const preview = document.querySelector('#preview');
preview.src = imageUrl;
// Revoke it after the image has loaded and is no longer needed.
preview.addEventListener('load', () => URL.revokeObjectURL(imageUrl), { once: true });Object URLs point to in-memory Blob data. Revoke them when the consumer is finished; otherwise a long-running page can retain memory unnecessarily.
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Decode a FileReader data URL
FileReader.readAsDataURL() uses the same structure. The header can contain a MIME type and the ;base64 marker. Decode only the portion after the first comma:
const input = document.querySelector('#file');
input.addEventListener('change', () => {
const file = input.files[0];
if (!file) return;
const reader = new FileReader();
reader.addEventListener('load', () => {
const dataUrl = reader.result;
const blob = dataUrlToBlob(dataUrl);
console.log(blob.type, blob.size);
});
reader.readAsDataURL(file);
});Splitting with split(',', 2) is intentional: it preserves the remainder as the payload. Percent-encoded, non-Base64 data must be decoded with decodeURIComponent; Base64 data must go through atob.
Use toBlob() for large images
toDataURL() constructs the complete encoded image in an in-memory JavaScript string. That can temporarily require substantially more memory than the final file, and very large strings can hit browser limits. MDN recommends toBlob() with URL.createObjectURL() instead.
function downloadCanvasBlob(canvas, type = 'image/png', quality) {
const extension = type === 'image/jpeg' ? 'jpg' :
type === 'image/webp' ? 'webp' : 'png';
canvas.toBlob((blob) => {
if (!blob) {
console.error('The browser could not encode this canvas.');
return;
}
const url = URL.createObjectURL(blob);
const link = document.createElement('a');
link.download = `canvas-image.${extension}`;
link.href = url;
link.click();
URL.revokeObjectURL(url);
}, type, quality);
}
downloadCanvasBlob(canvas, 'image/png');The callback can receive null, so handle that case. Revoke the object URL after the download has been initiated; if you keep the URL in an image, editor or preview, wait until that interaction ends before revoking it.
Upload without a Base64 round trip
canvas.toBlob(async (blob) => {
if (!blob) throw new Error('Encoding failed');
const response = await fetch('/api/images', {
method: 'POST',
headers: { 'Content-Type': blob.type || 'image/png' },
body: blob
});
if (!response.ok) throw new Error(`Upload failed: ${response.status}`);
}, 'image/png');This avoids embedding binary data in JSON and generally keeps memory use lower than creating a data URL first.
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A canvas is origin-clean only when every pixel source is permitted by the browser’s same-origin rules. If you draw an image, video or another canvas from a different origin without suitable CORS headers, the canvas becomes tainted. Reading pixels or exporting then throws a SecurityError.
try {
const dataUrl = canvas.toDataURL('image/png');
} catch (error) {
if (error.name === 'SecurityError') {
console.error('The canvas contains cross-origin pixels without CORS.');
}
throw error;
}For an image you control, request CORS before assigning src, and configure the image server to return an appropriate Access-Control-Allow-Origin response:
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const image = new Image();
image.crossOrigin = 'anonymous';
image.onload = () => {
ctx.drawImage(image, 0, 0);
const png = canvas.toDataURL('image/png');
};
image.onerror = () => console.error('Image could not be loaded with CORS');
image.src = 'https://images.example.com/photo.jpg';Setting crossOrigin after src is too late. The server must also opt in; a client-side flag cannot override a server that sends no CORS permission. If you do not control the remote server, proxy the resource through a server you control, subject to that resource’s terms and access controls. Do not attempt to bypass authentication or anti-forgery protections.
Empty output, unsupported formats and size limits
The returned value is data:,
A zero-sized canvas returns data:,. The same result is returned when the canvas exceeds the browser’s maximum canvas dimensions. Check dimensions before drawing and design large exports in tiles or at a reduced scale:
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if (canvas.width === 0 || canvas.height === 0) {
throw new Error('Canvas has no drawable area');
}
const result = canvas.toDataURL('image/png');
if (result === 'data:,') {
throw new Error('Canvas dimensions exceed this browser limit');
}Maximum dimensions vary by browser and device, so do not hard-code a universal limit. A large CSS element can also require more bitmap pixels than expected when you use a high device-pixel ratio; calculate the requested bitmap size deliberately.
The browser returns PNG despite your request
Unsupported MIME types fall back to PNG. Test the prefix:
const requested = canvas.toDataURL('image/webp', 0.8);
if (!requested.startsWith('data:image/webp;')) {
console.warn('WebP was not supported; the result may be PNG.');
}JPEG looks black or loses transparency
JPEG has no alpha channel. Paint an opaque background before exporting if transparent pixels should appear white (or another chosen color):
const copy = document.createElement('canvas');
copy.width = canvas.width;
copy.height = canvas.height;
const copyCtx = copy.getContext('2d');
copyCtx.fillStyle = '#fff';
copyCtx.fillRect(0, 0, copy.width, copy.height);
copyCtx.drawImage(canvas, 0, 0);
const jpeg = copy.toDataURL('image/jpeg', 0.85);Performance and reliability checklist
- Use
toDataURL()for small, immediate previews or downloads where a string is convenient. - Use
toBlob()plus an object URL for large images, repeated exports and uploads. - Keep the canvas origin-clean by setting
crossOriginbeforesrcand configuring the image server’s CORS response. - Call export after fonts, images and other asynchronous drawing operations have completed; otherwise the bitmap may be incomplete.
- Check for
data:,, anullBlob, unsupported MIME fallback andSecurityError. - Revoke object URLs when previews, downloads or editing sessions no longer need them.
- Choose PNG for transparency and crisp graphics; choose JPEG with a measured quality value for photographs.
- Use a user-initiated click for downloads, because browsers can restrict synthetic downloads outside user interaction.
Or skip the browser setup
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One GET request returns PNG, JPEG, WebP or PDF. The full option set includes full-page capture with lazy images loaded, CSS-selector element capture, dark mode, device presets and custom viewports, retina scale, PDF paper and page controls, custom CSS and JavaScript, pre-capture clicks, hidden selectors, selector/delay/network-idle waits, request and resource blocking, custom headers/cookies/user agent/Authorization, timezone and geolocation, transparent backgrounds, resizing, selectable-TTL caching, signed image links, asynchronous jobs with signed webhooks, bulk capture of up to 100 URLs per call, a usage API and an OpenAPI specification. Parameters used by other screenshot APIs also work, which can simplify migration.
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Frequently encountered errors
SecurityError: The operation is insecure
One or more drawn resources are cross-origin without CORS. Set crossOrigin before loading, configure the server, or use a permitted same-origin proxy.
InvalidCharacterError from atob()
You passed the data: header or a non-Base64 payload to atob. Split at the first comma and check for ;base64 first.
The download is tiny, blank or stale
Export only after drawing and asynchronous image/font loading finish. Confirm the canvas dimensions and inspect the bitmap in a preview before saving.
The tab becomes unresponsive
The encoded string is too large for a practical data-URL workflow. Switch to toBlob(), reduce bitmap dimensions, export less frequently, and release object URLs when finished.
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Does canvas.toDataURL() return Base64 only?
No. It returns a complete data URL. The Base64 payload follows the comma only when the header contains ;base64; the header also identifies the MIME type.
Can I decode a data URL with fetch()?
Yes. fetch(dataUrl) can produce a Blob, but splitting and decoding the URL yourself gives explicit control over MIME validation and malformed input handling.
What resolution does a canvas export have?
The pixel dimensions are the canvas bitmap’s width and height. Formats that carry resolution metadata use 96 dpi; that metadata does not add pixels.
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