Use canvas.toDataURL('image/png') when you need the image rendered by html2canvas. Use pdf.output('datauristring') after adding that image when you need the completed PDF. Both methods return a data URI; remove everything through the first comma only when the receiving API requires raw Base64.
Decide what you are encoding
There are two different artifacts in this workflow:
- The rendered image: html2canvas paints an element into an HTML canvas. Encode it with
canvas.toDataURL('image/png'). The result starts withdata:image/png;base64,. - The finished PDF: jsPDF builds a document and embeds the image. Encode it with
pdf.output('datauristring')(or its alias,dataurlstring). The result starts with a PDF media type and a Base64 marker.
Do not use the canvas string as a substitute for the PDF string. The canvas value represents only the rendered bitmap; the jsPDF value represents the completed PDF file.
Complete browser example
Install the libraries in your application (for example, with npm), then import them in a browser bundle:
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import html2canvas from 'html2canvas';
import { jsPDF } from 'jspdf';
async function encodeCapture() {
const element = document.querySelector('#capture');
if (!element) throw new Error('Missing #capture element');
// Wait for fonts before painting the element.
if (document.fonts?.ready) await document.fonts.ready;
const canvas = await html2canvas(element);
// Base64 data URI for the PNG rendered by html2canvas.
const imageDataUrl = canvas.toDataURL('image/png');
const pdf = new jsPDF();
pdf.addImage(imageDataUrl, 'PNG', 10, 10, 190, 0);
// Base64 data URI for the completed PDF.
const pdfDataUrl = pdf.output('datauristring');
// Only use this value if your endpoint explicitly wants raw Base64.
const rawBase64 = pdfDataUrl.slice(pdfDataUrl.indexOf(',') + 1);
return { canvas, imageDataUrl, pdfDataUrl, rawBase64 };
}
encodeCapture().then(({ pdfDataUrl }) => {
console.log(pdfDataUrl); // Keep the data:application/pdf;base64,... prefix when allowed
});The 190 value is the image width in the default jsPDF unit. A height of 0 asks jsPDF to preserve the image’s aspect ratio. Change the page, coordinates, and dimensions for your layout.
Convert an html2canvas canvas to Base64
Keep the data URI
canvas.toDataURL('image/png') synchronously serializes the canvas as a PNG data URI. Keeping the prefix is usually easiest because it tells the consumer both the media type and encoding:
const imageDataUrl = canvas.toDataURL('image/png');
imageElement.src = imageDataUrl;
await fetch('/upload-image', {
method: 'POST',
headers: { 'Content-Type': 'application/json' },
body: JSON.stringify({ image: imageDataUrl })
});You can request another image type supported by the browser, such as JPEG, but use the matching MIME type and jsPDF format. PNG is lossless and handles transparency; JPEG is typically appropriate for photographic content but does not preserve transparency.
Return only the Base64 payload
A raw Base64 endpoint does not accept the metadata prefix. Remove it at the first comma instead of splitting on a fixed string:
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const dataUrl = canvas.toDataURL('image/png');
const comma = dataUrl.indexOf(',');
if (comma === -1) throw new Error('Unexpected canvas data URL');
const rawImageBase64 = dataUrl.slice(comma + 1);Send the media type separately when the protocol requires it. Never strip the prefix merely because it looks cleaner; a data URI and a raw Base64 payload are different interface values.
Get a Base64 PDF from jsPDF
Use the string-returning output mode
Build the PDF first, then call pdf.output('datauristring'). dataurlstring is an alias that also returns a string. These modes are intended for programmatic handling.
const pdf = new jsPDF({ format: 'a4', unit: 'mm' });
pdf.addImage(imageDataUrl, 'PNG', 10, 10, 190, 0);
const pdfDataUrl = pdf.output('datauristring');
const pdfBase64 = pdfDataUrl.slice(pdfDataUrl.indexOf(',') + 1);
await fetch('/upload-pdf', {
method: 'POST',
headers: { 'Content-Type': 'application/json' },
body: JSON.stringify({ pdf: pdfDataUrl })
});Use pdfDataUrl when the server accepts a data URI. Use pdfBase64 only when the server contract says it wants the payload without the prefix. Encoding before addImage, or encoding the canvas and calling that value a PDF, produces the wrong artifact.
Prefer binary transport for large documents
Base64 is text wrapped around binary data and increases the amount of data your application must hold and transmit. If the receiving API supports binary uploads, ask jsPDF for an ArrayBuffer or Blob instead:
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const arrayBuffer = pdf.output('arraybuffer');
const blob = pdf.output('blob');
await fetch('/upload-pdf', {
method: 'POST',
headers: { 'Content-Type': 'application/pdf' },
body: blob
});Use Base64 for JSON fields, message queues, or APIs that explicitly require it. Use a Blob or ArrayBuffer when your transport can carry binary bytes directly.
Make the capture reliable
Wait for fonts and images
Capture only after the element has its final dimensions. document.fonts.ready helps prevent fallback-font text from being painted. For images, wait for loading before calling html2canvas:
async function waitForImages(root) {
const images = [...root.querySelectorAll('img')];
await Promise.all(images.map(img => {
if (img.complete) return img.decode?.().catch(() => {});
return new Promise(resolve => {
img.addEventListener('load', resolve, { once: true });
img.addEventListener('error', resolve, { once: true });
});
}));
}
const element = document.querySelector('#capture');
await document.fonts?.ready;
await waitForImages(element);
const canvas = await html2canvas(element);html2canvas works in modern evergreen browsers. It is a browser library, not a direct Node.js renderer; a Node process needs a browser environment if you want to run this workflow there.
Handle cross-origin resources
Images, fonts, or other resources loaded from another origin can taint the canvas. The page may appear correctly rendered, but reading pixels with toDataURL() can then fail with a security exception. Configure the asset server for appropriate CORS headers and load resources in a CORS-compatible way. When cross-origin content must be included, the html2canvas guidance is to use a proxy that accepts the URL and returns the resource as a Base64 data URI. Do not treat allowTaint as a way to bypass browser security; a tainted canvas remains unreadable.
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Common errors and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
SecurityError or “tainted canvas” |
A cross-origin image or font lacks usable CORS permission. | Serve the asset with the correct CORS headers, host it on the same origin, or route it through a proxy that returns a data URI. |
| PDF contains a blank or clipped page | The element was captured before layout settled, or its dimensions exceed the page. | Wait for fonts/images, inspect canvas.width/canvas.height, and calculate jsPDF dimensions and page breaks explicitly. |
| Server rejects the value as invalid Base64 | The server expects raw Base64 but received data:...;base64,, or vice versa. |
Confirm the API contract. Preserve the complete data URI when accepted; otherwise remove text through the first comma. |
| Only a PNG arrives when a PDF was expected | canvas.toDataURL() was sent instead of jsPDF output. |
Call pdf.output('datauristring') after pdf.addImage(). |
| Unexpected browser navigation | A navigation-oriented datauri/dataurl mode was selected. |
Use the string-returning datauristring or dataurlstring mode. |
| Works locally but fails in production | Production asset origins, CSP, authentication, or font loading differ. | Check browser console security errors and ensure every image/font is reachable with the same CORS and authentication behavior. |
Size, memory, and security considerations
- Canvas dimensions determine image memory. Capturing a very large, full-page element at high pixel density can exhaust browser memory before encoding.
- Base64 strings are convenient but costly for large payloads. Prefer a Blob or ArrayBuffer upload where possible.
- Do not put sensitive PDF data in URLs, logs, analytics events, or client-visible query strings. Data URIs can be copied like any other string.
- Validate the destination API’s maximum request size and timeout. Chunking or direct binary upload may be necessary for large files.
- Encode once, at the boundary that requires it. Repeated conversion between data URI, raw Base64, and binary increases memory use and creates opportunities to corrupt the payload.
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One GET request returns PNG, JPEG, WebP, or PDF:
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import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
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FAQ
Can I encode the canvas without creating a PDF?
Yes. Return the PNG (or another supported image type) from canvas.toDataURL(). It is an image data URI, not a PDF.
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Why does my Base64 value include a comma?
The comma separates the data-URI metadata from the encoded payload. Keep the entire value for a data-URI field; remove the prefix only for a raw-Base64 field.
Frequently Asked Questions
Can I encode the canvas without creating a PDF?
Yes. Return the PNG (or another supported image type) from canvas.toDataURL(). It is an image data URI, not a PDF.
Which jsPDF output mode should I use in application code?
Use datauristring or dataurlstring when you need a returned string. Choose arraybuffer or blob when your transport accepts binary data.
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Why does my Base64 value include a comma?
The comma separates the data-URI metadata from the encoded payload. Keep the entire value for a data-URI field; remove the prefix only for a raw-Base64 field.
The Bottom Line
Encode the rendered image with canvas.toDataURL(), but encode the completed PDF with pdf.output('datauristring'). Strip the data-URI prefix only when the receiving API explicitly requires raw Base64.
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