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This workflow keeps image data binary, lets your server enforce size and type limits, and works with any storage system your application chooses. The endpoint URL, authentication method, maximum size, storage name, and response format are application decisions—not html2canvas features.
Complete browser-to-server example
Install the package used by your project and import it. The current package name is @html2canvas/html2canvas; use the version and package manager standardized by your application.
npm install @html2canvas/html2canvasThe following function captures #capture as a PNG and uploads it to /api/screenshots. Replace that path with your authenticated endpoint.
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import html2canvas from '@html2canvas/html2canvas';
async function uploadScreenshot() {
const element = document.querySelector('#capture');
if (!element) throw new Error('Capture target not found');
const canvas = await html2canvas(element, {
backgroundColor: '#ffffff',
scale: window.devicePixelRatio,
useCORS: true
});
const blob = await new Promise((resolve, reject) =>
canvas.toBlob(result => result ? resolve(result) : reject(new Error('Canvas export failed')), 'image/png')
);
const form = new FormData();
form.append('screenshot', blob, 'screenshot.png');
const response = await fetch('/api/screenshots', {
method: 'POST',
body: form,
credentials: 'same-origin'
});
if (!response.ok) throw new Error(`Upload failed: ${response.status}`);
return response.json();
}Call uploadScreenshot() from a button handler or another user action. credentials: 'same-origin' sends same-site cookies; use the authentication scheme required by your API if it is different.
Why the Blob step matters
canvas.toBlob() produces a binary file-like object without expanding the image into a long text string. The callback can return null, so the example rejects explicitly instead of uploading an invalid value. The filename passed to form.append is only a client hint; the server must not trust it for storage or type validation.
Do not set multipart Content-Type yourself
When fetch receives a FormData body, the browser adds a boundary such as multipart/form-data; boundary=…. Setting Content-Type: multipart/form-data manually omits that boundary in many clients and causes parsers to reject the request.
Capture the right element and control the output
Pass the specific DOM element you want, not the selector string. html2canvas reconstructs the element from DOM nodes and styles; it does not capture the browser compositor’s final output. Unsupported CSS, plugins, and browser-specific behavior can therefore produce an image that differs from what the user sees. The project describes the result as a browser-side “screenshot” and cautions that it may not be 100% accurate to the real representation.
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Quality, crop, and viewport options
scale: The documented default is the browser’s device-pixel ratio. A higher value can sharpen output but increases memory use, processing time, and upload size.backgroundColor: Set a known color such as#fffffffor predictable PNGs, or usenullwhen transparency is required and the downstream format supports it.x,y,width,height: Capture a region rather than the whole element when a full-page image is unnecessary.windowWidthandwindowHeight: Choose the viewport used to evaluate media queries. This is useful when the capture must represent a fixed responsive breakpoint.imageTimeout: Increase the image-loading timeout for slow assets, or disable it only when your application accepts potentially long captures.data-html2canvas-ignoreandignoreElements: Exclude controls, transient notices, or sensitive fields from the image.
For example, this configuration captures a fixed-width region, hides elements marked for exclusion, and keeps a transparent background:
const canvas = await html2canvas(document.querySelector('#capture'), {
x: 0,
y: 0,
width: 1200,
height: 800,
windowWidth: 1200,
windowHeight: 900,
backgroundColor: null,
ignoreElements: element => element.matches('[data-private], .temporary-button'),
imageTimeout: 20000,
useCORS: true
});Do not combine an extremely large scale with a large element unless you have measured browser memory and request-size limits. A capture can fail before the network request is made if the canvas cannot be allocated.
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Upload alternatives: PNG, JPEG, and base64
Binary PNG or JPEG with toBlob
PNG is lossless and is a sensible default for interface screenshots, text, and transparency. If transparency is not needed and smaller files are more important, request JPEG and provide a quality value:
const blob = await new Promise((resolve, reject) =>
canvas.toBlob(
file => file ? resolve(file) : reject(new Error('Canvas export failed')),
'image/jpeg',
0.85
)
);Match the filename extension to the MIME type you request. The server should still inspect the decoded bytes rather than trusting either value.
Data URL for APIs that explicitly require base64
The data-URL form is convenient when an API accepts JSON text, but it is less efficient for ordinary file uploads because binary data is expanded into encoded text.
const dataUrl = canvas.toDataURL('image/png');
await fetch('/api/screenshots/base64', {
method: 'POST',
headers: {'Content-Type': 'application/json'},
body: JSON.stringify({ image: dataUrl })
});Only remove the data:image/png;base64, prefix when the receiving API explicitly requires raw base64. Otherwise preserve the complete data URL or use a Blob endpoint.
Design the receiving endpoint safely
Your server is receiving untrusted input. A robust endpoint should:
- Authenticate and authorize the request before accepting the file.
- Apply a request-size limit and a decoded-image-size limit.
- Parse the multipart field named
screenshot(or the name you selected). - Verify the actual image type by decoding the bytes; do not rely on the filename or declared MIME type.
- Generate a safe storage key rather than using the client-provided filename.
- Store the object with the access policy your application requires.
- Return a small, explicit JSON result, such as an application-generated ID and URL, or a clear error code.
Keep the upload response separate from image processing if resizing, scanning, or moderation may take time. A short success response also prevents clients from depending on storage-provider-specific metadata.
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Example response contract
// success
{
"id": "generated-by-your-server",
"url": "https://your-app.example/images/generated-key.png"
}
// failure
{
"error": "image_too_large"
}These values are examples of an application-defined contract. They are not supplied by html2canvas and must be implemented by your API.
Cross-origin images and blank or tainted exports
Cross-origin images are the most common reason an export is blank, incomplete, or unreadable. Setting useCORS: true helps only when the image server sends suitable CORS headers. If the remote server does not permit the browser origin, the image cannot be safely read back from the canvas.
Use CORS headers when you control the image host
Configure the image response to allow the requesting origin, then keep useCORS: true. The exact header policy depends on whether credentials are involved. Avoid allowing every origin by habit when the asset is private.
Use a controlled proxy when the source cannot provide CORS
The documented proxy option can point to a proxy under your control. That proxy must fetch the resource and return it in a form the browser can read. Protect it against open-proxy abuse, restrict allowed destinations, apply timeouts and response-size limits, and avoid forwarding private network requests.
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Understand iframe limits
A cross-origin iframe cannot be rendered because its contentDocument is inaccessible to the page. You cannot bypass that browser security boundary by changing html2canvas options. Capture content that your page is allowed to read, or generate the image in a server-side browser that has access under an appropriate authorization model.
Prevent an already-tainted canvas
If any cross-origin content without usable CORS rules has been drawn, reading it with toBlob or toDataURL can throw a security error. Fix the source, proxy it safely, or exclude the asset before drawing; exporting after the fact cannot untaint the canvas.
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Common failures and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
Capture target not found |
The selector ran before the element existed or is misspelled. | Call the function after rendering, verify the selector in developer tools, and handle a missing target explicitly. |
| Blank image or missing remote images | Cross-origin resources lack suitable CORS headers or have not finished loading. | Enable useCORS, configure the asset host, or use a secured proxy; increase imageTimeout only for slow but permitted assets. |
| Security error while exporting | The canvas was tainted by unreadable cross-origin content. | Resolve CORS/proxy access before drawing. Re-render after fixing the source. |
| Text, shadows, or layout differs from the page | html2canvas reconstructs supported DOM and CSS rather than capturing compositor pixels. | Check supported styles, simplify the capture, or use browser automation when pixel fidelity to the rendered page is mandatory. |
| Upload returns 400 or the server says no file was found | Field name mismatch or malformed multipart boundary. | Make the server read screenshot, pass FormData directly, and remove any manually set multipart Content-Type. |
| 413 Payload Too Large or browser memory failure | Element dimensions or scale produce an oversized image. |
Crop with width/height, lower scale, choose JPEG where appropriate, and align client and server limits. |
toBlob returns null |
The browser could not encode the requested format or the canvas is unusable. | Reject the result, verify canvas dimensions and format, and retry with a supported type. |
| Capture waits indefinitely | Slow assets, never-ending network activity, or an overly generous timeout. | Set a bounded imageTimeout, ensure assets resolve, and cancel the surrounding operation when your UI no longer needs it. |
When html2canvas is the wrong capture method
html2canvas is useful when the capture must happen in the user’s browser, should include the current DOM state, and should not require sending the page to a server for rendering. It also lets your application hide sensitive or transient elements before export.
A browser-automation screenshot is a better fit when you need pixels close to the browser’s compositor output, must capture cross-origin content from a controlled server context, or need generation without user interaction. That approach adds a server/runtime, browser lifecycle, and authorization design. Neither method removes the need to validate uploaded output if a client sends the file to your API.
Performance, reliability, and privacy checklist
- Capture only the smallest element and region that meets the requirement.
- Use the device-pixel-ratio default first; increase
scaleonly after checking memory and upload size. - Wait until fonts, images, and application data needed by the target are ready.
- Exclude controls and private fields with
data-html2canvas-ignoreorignoreElements. - Set bounded client and server timeouts and show a retryable error to the user.
- Keep authentication on the upload endpoint and apply CSRF protection when cookie authentication is used.
- Log a request ID and server-side validation result, not the image contents or sensitive form values.
- Test responsive breakpoints, slow images, blocked images, long pages, and browsers your users actually support.
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FAQ
Can I upload the canvas directly instead of creating a Blob?
No. FormData needs a file-like value, so convert the canvas with toBlob. Use a data URL only when the receiving API explicitly expects base64 text.
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Will html2canvas capture a cross-origin iframe?
No. A cross-origin iframe’s document is inaccessible to the page, so html2canvas cannot reconstruct it.
Should screenshots be stored under the filename sent by the browser?
No. Treat the filename as untrusted metadata and generate a storage key on the server after validating the decoded image.
Is a higher scale always better?
No. It can improve detail while increasing memory use and upload size; choose the smallest value that meets your display or print requirement.
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Can I upload the canvas directly instead of creating a Blob?
No. Convert it with toBlob for a FormData file upload; use a data URL only when an API specifically requires base64 text.
Will html2canvas capture a cross-origin iframe?
No. Browser same-origin rules prevent access to the iframe document.
Should I trust the filename sent by the browser?
No. Validate decoded bytes and generate a server-side storage key.
Is a higher scale always better?
No. Higher scale can improve detail but increases memory use and upload size.
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