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You can resize and re-encode a user-selected image entirely in the browser: load it, draw it to a correctly sized canvas, then export a Blob with canvas.toBlob(). No server is needed for this processing step. If your app later sends the result to a server, that upload is separate.
How browser-based image compression works
A canvas export can change an image’s pixel dimensions, its encoding format, or both. To resize, set the canvas width and height to the output dimensions before drawing the image. To re-encode without resizing, draw it at its existing dimensions and export in the format you want.
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HTMLCanvasElement.toBlob() produces a Blob and accepts an optional MIME type plus a quality value from 0 to 1 for formats with lossy encoders. A quality value is an encoder setting, not a promised file size or visual result. Check the resulting file’s byte size and inspect its appearance rather than assuming a particular reduction. MDN documents the export options and behavior.
Resize and export a selected image
This example takes a local image selected by the user, caps its longest side at 1600 pixels, preserves its aspect ratio, and requests JPEG output. The limit and quality value are example settings; adjust them to suit your application.
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- Load and decode the local file. Create a temporary object URL for the selected file, then wait for the image to decode.
- Set the output dimensions. Calculate the scale so neither side exceeds the chosen maximum, then set the canvas dimensions before drawing.
- Export and verify. Request JPEG with a quality setting, handle a missing result, and inspect the returned Blob’s type.
- Preview or download the result. Use an object URL and revoke it when the preview or download lifecycle is finished.
const input = document.querySelector('input[type="file"]');
const canvas = document.querySelector('canvas');
const maxSide = 1600;
input.addEventListener('change', async () => {
const file = input.files?.[0];
if (!file) return;
const sourceUrl = URL.createObjectURL(file);
const image = new Image();
try {
image.src = sourceUrl;
await image.decode();
const scale = Math.min(1, maxSide / Math.max(image.naturalWidth, image.naturalHeight));
const width = Math.round(image.naturalWidth * scale);
const height = Math.round(image.naturalHeight * scale);
canvas.width = width;
canvas.height = height;
const context = canvas.getContext('2d');
if (!context) throw new Error('Canvas 2D context is unavailable');
context.drawImage(image, 0, 0, width, height);
const blob = await new Promise((resolve, reject) => {
canvas.toBlob(result => {
if (result) resolve(result);
else reject(new Error('Canvas image export failed'));
}, 'image/jpeg', 0.85);
});
console.log('Actual output type:', blob.type);
console.log('Output size in bytes:', blob.size);
const previewUrl = URL.createObjectURL(blob);
const preview = document.querySelector('#preview');
preview.src = previewUrl;
preview.onload = () => URL.revokeObjectURL(previewUrl);
} catch (error) {
console.error(error);
} finally {
URL.revokeObjectURL(sourceUrl);
}
});The example assumes the page contains a file input, a canvas, and an image element with id="preview". In production, show a user-facing error instead of relying only on console.error, and manage the preview URL until the preview is replaced or no longer needed. Object URLs keep the binary data in a practical form; they should not be left active indefinitely.
Choose a format and quality deliberately
| Choice | What to expect | Important consideration |
|---|---|---|
| PNG | Browsers are required to support PNG canvas export. | The quality parameter is not a lossy-quality control for PNG. PNG preserves transparency, but the output size depends on the image and encoding. |
| JPEG | Supported by many browsers; quality can be supplied from 0 to 1. | JPEG does not preserve transparency. If the source has transparent pixels, composite onto an intentional background or choose a format that supports transparency. |
| WebP | Supported by many browsers; quality can be supplied from 0 to 1. | Support for optional export formats is not universal. Inspect blob.type instead of assuming the requested type was produced. |
If the requested MIME type is omitted or unsupported, canvas export falls back to PNG. That makes checking blob.type important when an application depends on a particular format. The MDN toBlob() reference describes this behavior. There is no universal quality value that guarantees a particular file size or appearance; compare the actual output and look for artifacts at the intended display size.
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Use Blob and object URLs for practical output
toBlob() gives you binary output that can be previewed with URL.createObjectURL(), downloaded, or passed to an upload flow. Revoke temporary object URLs once they are no longer needed.
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Handle remote images and canvas security
A local file selected by the user can be loaded through an object URL as in the example. A remote image has an additional restriction: drawing an image from another origin without the required CORS permission taints the canvas. An attempt to export a tainted canvas with toBlob() or another readback operation can throw a SecurityError.
For a remote image, set image.crossOrigin before assigning its src, and the image server must return an appropriate Access-Control-Allow-Origin response header. The client-side property alone cannot grant access if the server does not permit it. This is a browser security boundary, not a compression failure. See MDN’s guide to cross-origin images in canvas.
Quick Recap
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Common failures and what to check
- The export callback returns
null: treat it as an export failure and show an error; the API does not guarantee a Blob on every call. - The output format differs from the request: inspect
blob.type. Unsupported requested formats fall back to PNG. - The export throws
SecurityError: check whether the canvas includes a cross-origin image and whether that image server permits CORS. - The output is not much smaller: confirm that the canvas dimensions were reduced if resizing was intended, check the actual output type and byte size, and review image quality. The quality input alone does not guarantee a size reduction.
- Transparent areas become opaque: JPEG cannot retain transparency. Use a transparency-capable output format or draw onto a deliberate background before export.
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