navigator.mediaDevices.getDisplayMedia() returns a live MediaStream, not an image file. To save one screenshot, request a user-selected display, read its video track with ImageCapture.grabFrame(), draw the resulting ImageBitmap on a canvas, and export the canvas with toBlob(). Always start the request from a user gesture, handle cancellation and browser errors, and stop every track when finished.
The complete browser workflow
The Screen Capture API is designed for sharing or recording a live stream. A still-image workflow has four stages:
- Call
getDisplayMedia()from a button click or another transient user-activation handler. - Take the selected stream’s video track.
- Use
ImageCapture.grabFrame()to obtain oneImageBitmap. - Draw it to a canvas and encode the canvas as PNG, JPEG or another supported image type.
The browser-controlled chooser lets the person select a display, window or tab. Script options can influence capture output, but cannot silently select or pre-filter a source on the user’s behalf.
Minimal runnable example
<button id="capture">Capture screenshot</button>
<a id="download" hidden>Download PNG</a>
<img id="preview" alt="Captured screen preview">
<script>
const button = document.querySelector('#capture');
const download = document.querySelector('#download');
const preview = document.querySelector('#preview');
let previousUrl;
button.addEventListener('click', async () => {
let stream;
let bitmap;
try {
stream = await navigator.mediaDevices.getDisplayMedia({
video: true,
audio: false,
preferCurrentTab: true
});
const [track] = stream.getVideoTracks();
if (!track) throw new Error('No video track was returned.');
bitmap = await new ImageCapture(track).grabFrame();
const canvas = document.createElement('canvas');
canvas.width = bitmap.width;
canvas.height = bitmap.height;
const context = canvas.getContext('2d');
context.drawImage(bitmap, 0, 0);
const blob = await new Promise((resolve, reject) => {
canvas.toBlob(result => result ? resolve(result) : reject(new Error('PNG encoding failed.')), 'image/png');
});
if (previousUrl) URL.revokeObjectURL(previousUrl);
previousUrl = URL.createObjectURL(blob);
preview.src = previousUrl;
download.href = previousUrl;
download.download = 'screen-capture.png';
download.hidden = false;
} catch (error) {
console.error(error);
alert(`Screenshot failed: ${error.name || 'Error'}${error.message ? ` — ${error.message}` : ''}`);
} finally {
if (stream) stream.getTracks().forEach(track => track.stop());
if (bitmap) bitmap.close();
}
});
</script>The example revokes the previous object URL before replacing it, closes the bitmap, and stops all stream tracks. Those cleanup steps prevent an abandoned capture session or image resource from remaining active.
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Choosing capture options safely
A typical request is { video: true, audio: false }. Audio is unnecessary for a still and can trigger an additional sharing decision. preferCurrentTab: true can indicate that the current tab is preferred where the browser supports the option, but it does not force that choice.
getDisplayMedia() requires video. Passing video: false is invalid. Constraints and hints apply after the person chooses a source; they do not remove the browser’s chooser or let an application select a particular monitor or window without consent. The permission prompt can appear again for every request rather than being silently reused.
Capture dimensions and image format
The bitmap dimensions come from the selected track. Set the canvas to bitmap.width and bitmap.height to avoid stretching. PNG is lossless and suitable for UI text. For photographs or smaller files, encode as JPEG or WebP where the browser supports it:
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const blob = await new Promise((resolve, reject) => {
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});Replace the illustrative quality value with a number from 0 to 1 (for example, 0.9); PNG ignores the quality argument. If you need a deterministic file type across browsers, use image/png.
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The ordinary flow captures the entire selected surface. Newer capture extensions can narrow the result after selection, but their support is more limited than basic screen sharing.
Element Capture
Element Capture isolates a chosen rendered DOM element and its descendants. Content from other page layers that overlaps the element is excluded. The documented pattern creates a restriction target from an element and calls track.restrictTo() before grabbing a frame. It requires support for both Element Capture and ImageCapture.grabFrame().
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Region Capture
Region Capture crops the tab to an element’s bounding rectangle. Unlike Element Capture, content from another layer that overlaps that rectangle can remain visible. Use Region Capture for geometric cropping; use Element Capture when DOM isolation and privacy are more important.
MDN describes Element and Region Capture as desktop-browser capabilities. Check current compatibility tables for the exact browser versions you support, and provide a fallback to whole-surface capture or a normal DOM screenshot method when the extensions are unavailable.
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User activation
Invoke getDisplayMedia() directly inside a click, pointer or keyboard handler. Calling it later from an unrelated timer or background task can produce InvalidStateError. Do not attempt to bypass the chooser: screen sharing can expose passwords, private messages and other sensitive material.
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Permissions Policy and iframes
If a site uses Permissions Policy, the parent document must allow display capture. For an embedded page, the parent can use:
<iframe src="/capture.html" allow="display-capture"></iframe>The documented default allowlist for the display-capture directive is self. An iframe allowance enables the policy but never replaces the user’s source-selection prompt.
Error handling and troubleshooting
| Error or symptom | Likely cause | Fix |
|---|---|---|
InvalidStateError |
The call was not made during transient user activation, or the document is not in a usable state. | Start the request synchronously in a button handler and avoid calling it from a timer. |
NotAllowedError |
The user denied capture, the browser blocked it, or an enclosing policy disallows it. | Explain why access is needed, retry from a fresh gesture, and check iframe allow and Permissions Policy headers. |
NotFoundError |
No capturable source was available. | Ask the user to select an available display, window or tab and verify that the operating system permits screen recording. |
NotReadableError |
The selected source could not be read because of an OS, hardware or browser failure. | Close competing screen-recording software, retry, and test another source or browser. |
| No image after selection | The stream has no video track, the track ended, or frame capture is unsupported. | Check getVideoTracks(), listen for ended, test ImageCapture support, and show a fallback message. |
| Black or unexpected content | The user selected a different surface, a protected window, or a region with overlapping content. | Describe the required source before opening the chooser; use Element Capture rather than Region Capture when supported and privacy matters. |
| Memory growth after repeated captures | Old blob URLs, bitmaps or tracks were not released. | Call URL.revokeObjectURL(), bitmap.close(), and track.stop() in cleanup code. |
Recording versus one still image
Use the stream directly with MediaRecorder when the requirement is a video or live broadcast. Use grabFrame() and canvas encoding when you need a single PNG, periodic stills, thumbnails or an uploadable image. A stream remains live until its tracks end or you stop them; a bitmap is a detached snapshot that should be closed after use.
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Performance and reliability considerations
- Canvas memory grows with pixel count. A 4K capture requires substantially more memory than a laptop-sized tab, so resize only after deciding what detail your application needs.
- Do not keep a stream open while waiting for a user to download one frame. Capture once, stop the tracks and release the bitmap.
- For repeated frames, reuse a canvas where practical, but still close each
ImageBitmapand stop the stream when the session ends. - Expect the user to switch windows, minimize a source or stop sharing from browser controls. Monitor
track.onendedand disable capture controls when the track ends. - Use HTTPS (or a secure localhost origin) and test the exact desktop browsers your users run. Optional Element and Region Capture support varies.
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open("shot.webp", "wb").write(r.content)Node.js:
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Frequently Asked Questions
Can getDisplayMedia() save a PNG by itself?
No. It returns a live MediaStream. You must extract a video frame, draw it to a canvas and encode the canvas.
Can a site choose a monitor without showing a chooser?
No. Browser security keeps source selection under user control, and permission cannot be silently reused.
Is screen capture available in an iframe?
It can be, provided the embedding policy allows display capture, such as allow="display-capture"; the user still approves the source.
Why use Element Capture instead of Region Capture?
Element Capture isolates the element’s DOM content. Region Capture crops a rectangle and may include content from another layer that overlaps it.
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