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You can resize and re-encode a user-selected image entirely in the browser: load the local file, draw it onto a correctly sized canvas, and export the canvas with toBlob(). No server is needed for that compression step. If you later upload the resulting Blob, that is a separate operation.
What browser-side canvas compression does
A canvas export creates a new image from the pixels drawn onto the canvas. Setting the canvas width and height before drawing lets you resize the image as part of that export. Requesting a lossy format such as JPEG or WebP can also re-encode the pixels, but the quality setting does not guarantee a particular file size or visual result. [MDN: toBlob()]
The example below accepts a local image, limits its longest side, preserves its aspect ratio, exports a JPEG Blob, and provides a preview and download link.
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Compress and resize a local image
Example: file input, canvas export, preview, and download
<input id="image-file" type="file" accept="image/*">
<label>Maximum side in pixels
<input id="max-side" type="number" min="1" value="1600">
</label>
<button id="compress" type="button" disabled>Compress image</button>
<p id="status">Choose an image to begin.</p>
<img id="preview" alt="Compressed image preview">
<a id="download" hidden>Download compressed image</a>
<script>
const fileInput = document.querySelector("#image-file");
const maxSideInput = document.querySelector("#max-side");
const button = document.querySelector("#compress");
const status = document.querySelector("#status");
const preview = document.querySelector("#preview");
const download = document.querySelector("#download");
let sourceUrl = null;
let previewUrl = null;
fileInput.addEventListener("change", () => {
if (sourceUrl) URL.revokeObjectURL(sourceUrl);
if (previewUrl) URL.revokeObjectURL(previewUrl);
preview.removeAttribute("src");
download.hidden = true;
download.removeAttribute("href");
button.disabled = !fileInput.files.length;
const file = fileInput.files[0];
if (!file) {
sourceUrl = null;
status.textContent = "Choose an image to begin.";
return;
}
sourceUrl = URL.createObjectURL(file);
status.textContent = "Image selected. Choose Compress image.";
});
button.addEventListener("click", async () => {
const file = fileInput.files[0];
if (!file || !sourceUrl) return;
const maxSide = Number(maxSideInput.value);
if (!Number.isFinite(maxSide) || maxSide < 1) {
status.textContent = "Enter a maximum side of at least 1 pixel.";
return;
}
button.disabled = true;
status.textContent = "Processing image…";
try {
const image = new Image();
image.src = sourceUrl;
await image.decode();
const scale = Math.min(1, maxSide / Math.max(image.naturalWidth, image.naturalHeight));
const width = Math.max(1, Math.round(image.naturalWidth * scale));
const height = Math.max(1, Math.round(image.naturalHeight * scale));
const canvas = document.createElement("canvas");
canvas.width = width;
canvas.height = height;
const context = canvas.getContext("2d");
if (!context) throw new Error("Canvas 2D is unavailable in this browser.");
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("The browser could not create an image Blob."));
}, "image/jpeg", 0.85);
});
if (previewUrl) URL.revokeObjectURL(previewUrl);
previewUrl = URL.createObjectURL(blob);
preview.src = previewUrl;
download.href = previewUrl;
download.download = file.name.replace(/.[^.]+$/, "") + ".jpg";
download.hidden = false;
const sizeKB = (blob.size / 1024).toFixed(1);
status.textContent = `Created ${width} × ${height} ${blob.type || "image"} (${sizeKB} KB).`;
} catch (error) {
status.textContent = error instanceof Error ? error.message : "Image processing failed.";
} finally {
button.disabled = false;
}
});
window.addEventListener("pagehide", () => {
if (sourceUrl) URL.revokeObjectURL(sourceUrl);
if (previewUrl) URL.revokeObjectURL(previewUrl);
});
</script>How the example works
- The file input gives the page a local file.
URL.createObjectURL()creates a temporary URL so anImagecan load it without sending it to a server. image.decode()waits for the image to be decoded before its dimensions and pixels are used.- The scale calculation caps the longest side at the entered maximum and never enlarges a smaller image. The canvas dimensions are set to the resulting pixel width and height before
drawImage(). toBlob()asks the browser to encode the canvas as JPEG at quality0.85. The callback can returnnull, so the example treats that as an error.- The returned Blob is previewed and linked for download through an object URL. The page revokes previous temporary URLs when they are no longer needed and when the document is left.
Choose an output format and quality
The requested MIME type is a request, not a guarantee. PNG export is required by browsers; if the requested type is omitted or unsupported, the browser exports PNG instead. JPEG and WebP are supported by many browsers, but you should inspect the actual blob.type before naming or handling the file. MDN documents the supported behavior of toBlob().
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- JPEG: A lossy option for photographs when transparency is not needed. The example uses quality 0.85 as an encoder input, not as a promise of a particular reduction.
- WebP: Another lossy option where supported. Check the returned Blob type and provide a fallback or tell the user if the requested format was not produced.
- PNG: A lossless-format option and the browser-required fallback. The quality parameter is not a lossy-quality control for PNG.
JPEG cannot preserve transparent pixels. If the source has transparency, choose an output format that can retain it, or deliberately draw the source over a chosen background before exporting to JPEG. After export, compare the output’s byte size and appearance; dimensions, image content, format, and encoder behavior all affect the result, and the quality number alone does not predict file size.
Why use a Blob instead of a data URL?
For ordinary previews and downloads, a Blob paired with URL.createObjectURL() is usually a better fit than toDataURL(). A data URL materializes the entire encoded image as an in-memory string, which can create performance problems with large images or exceed URL length limits. Use it only when a string representation is specifically useful. Revoke object URLs when their preview or download lifecycle is over. [MDN: toDataURL()] [MDN: toBlob()]
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When a remote image cannot be exported
A canvas that draws an image from another origin may become tainted unless cross-origin use is approved. Exporting a tainted canvas with toBlob() or another read operation can throw a SecurityError. This is a browser security boundary, not a compression failure. [MDN: cross-origin images in a canvas]
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- Set the image element’s
crossOriginproperty before assigning itssrc, for example:image.crossOrigin = "anonymous";. - The remote image server must also return an appropriate
Access-Control-Allow-Originresponse header for your page’s origin (or an allowed wildcard where appropriate). - If the server does not allow your origin, changing client-side code cannot grant permission. Use a user-selected local file or arrange for the image server to enable CORS.
Common failures and fixes
- The output is PNG instead of JPEG or WebP: Inspect
blob.type. The requested optional format may not be supported, in which case export falls back to PNG. - The export callback produces no Blob: Handle a
nullresult as an encoding failure and show a useful message instead of creating a preview URL. - The file is not smaller: A quality value does not set a target size. Try a smaller canvas dimension or a different supported format, then inspect the resulting size and visual quality.
- Transparent areas turn opaque or dark: JPEG does not carry transparency. Preserve it with a suitable format or composite onto an intentional background.
- Export throws a security error: Check whether the canvas includes an image from another origin and whether both the image request and server response permit CORS.
For the API’s normative behavior, see the WHATWG HTML Standard.
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