To preview an image before uploading it, read the first selected File from an <input type="file">, create a temporary object URL with URL.createObjectURL(file), and assign that URL to an <img> element. The browser displays the local image without sending it to your server. Release each object URL with URL.revokeObjectURL() when the preview is replaced or removed.
This pattern is lightweight, works for single or multiple files, and can be reused for drag-and-drop. The complete examples below validate the selection, handle errors, preserve accessibility, and clean up browser resources.
Basic single-image preview
Start with a labeled file control and an image that is hidden until a valid selection exists. The accept attribute guides the picker, but it is not a complete security check; JavaScript should still inspect the selected file and your server must validate the upload independently.
<label for="image-input">Choose an image</label>
<input id="image-input" type="file" accept="image/*">
<p id="image-error" role="alert" hidden></p>
<img id="image-preview" alt="Selected image preview" hidden>
<script>
const input = document.querySelector("#image-input");
const preview = document.querySelector("#image-preview");
const error = document.querySelector("#image-error");
let previewUrl;
input.addEventListener("change", () => {
const file = input.files?.[0];
if (previewUrl) {
URL.revokeObjectURL(previewUrl);
previewUrl = undefined;
}
preview.removeAttribute("src");
preview.hidden = true;
error.hidden = true;
error.textContent = "";
if (!file) {
return;
}
if (!file.type.startsWith("image/")) {
error.textContent = "Choose an image file.";
error.hidden = false;
return;
}
previewUrl = URL.createObjectURL(file);
preview.src = previewUrl;
preview.hidden = false;
});
function clearImagePreview() {
if (previewUrl) {
URL.revokeObjectURL(previewUrl);
previewUrl = undefined;
}
preview.removeAttribute("src");
preview.hidden = true;
input.value = "";
}
</script>The files property is a FileList; optional chaining makes an empty selection safe. A File supplies metadata such as name, size, and type. URL.createObjectURL(file) returns a blob URL that can be used as an image source. See MDN’s Using files from web applications and File references.
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Do not revoke the URL immediately in an image load handler if the image remains available. MDN notes that early revocation can break interactions such as opening the image in a new tab or saving it. Revoke it when replacing the file, clearing the component, or unmounting the page instead. Each call to createObjectURL() creates a new URL, even for the same object; the blob URL reference explains the memory implications.
What happens when the user selects a file
- The picker returns a
FileListthroughinput.files. - Your handler rejects an empty selection or a type your interface does not support.
- The browser creates a temporary URL pointing at the local file; it does not upload the bytes.
- The image element decodes and displays that local resource.
- When the preview is no longer needed,
URL.revokeObjectURL()releases the URL reference.
The URL is only useful in the browser context that created it. It is not a permanent URL to send to your API. When the user submits the form, send the original File in FormData (or use your chosen upload protocol).
Validation that helps users without pretending to be security
Use accept as a picker hint
accept="image/*" asks the file picker to emphasize image formats. Users can still select unexpected content in some environments, so keep the JavaScript check. The MDN file-input documentation describes accept as a user-interface filter rather than proof of file contents.
Check type, size, and dimensions when appropriate
A MIME-type check such as file.type.startsWith("image/") gives immediate feedback, but a MIME string alone does not prove that the bytes are a valid image. Enforce authoritative type, size, and content checks on the server before storing or processing the upload. You can check a client-side size limit to avoid an unpleasant preview attempt:
const MAX_BYTES = 10 * 1024 * 1024;
if (file.size > MAX_BYTES) {
error.textContent = "Choose an image no larger than 10 MB.";
error.hidden = false;
return;
}To inspect dimensions after loading, listen for preview.onload and read preview.naturalWidth and preview.naturalHeight. Treat those values as UI guidance, not a replacement for server-side validation.
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Supporting multiple images
Add multiple, create one object URL per accepted file, and keep the URLs in an array so every one can be revoked. The following example also reports rejected files.
<label for="images">Choose images</label>
<input id="images" type="file" accept="image/*" multiple>
<p id="multi-error" role="alert" hidden></p>
<div id="gallery" aria-live="polite"></div>
<script>
const filesInput = document.querySelector("#images");
const gallery = document.querySelector("#gallery");
const multiError = document.querySelector("#multi-error");
let galleryUrls = [];
function clearGallery() {
galleryUrls.forEach(url => URL.revokeObjectURL(url));
galleryUrls = [];
gallery.replaceChildren();
}
filesInput.addEventListener("change", () => {
clearGallery();
multiError.hidden = true;
multiError.textContent = "";
const rejected = [];
for (const file of filesInput.files ?? []) {
if (!file.type.startsWith("image/")) {
rejected.push(file.name);
continue;
}
const url = URL.createObjectURL(file);
galleryUrls.push(url);
const figure = document.createElement("figure");
const image = document.createElement("img");
image.src = url;
image.alt = file.name;
image.loading = "lazy";
const caption = document.createElement("figcaption");
caption.textContent = file.name;
figure.append(image, caption);
gallery.append(figure);
}
if (rejected.length) {
multiError.textContent = `Skipped non-image files: ${rejected.join(", ")}`;
multiError.hidden = false;
}
});
// Call clearGallery() when this component is destroyed or reset.
</script>Track URLs independently from DOM nodes. Removing an <img> element does not itself communicate that the blob URL is no longer needed. MDN’s file-input examples show the same thumbnail-and-cleanup approach.
Drag-and-drop uses the same preview function
A drop event exposes files through event.dataTransfer.files. Reuse the rendering logic rather than maintaining a separate image-decoding path.
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dropZone.addEventListener("dragover", event => {
event.preventDefault();
dropZone.classList.add("is-over");
});
dropZone.addEventListener("dragleave", () => {
dropZone.classList.remove("is-over");
});
dropZone.addEventListener("drop", event => {
event.preventDefault();
dropZone.classList.remove("is-over");
const droppedFiles = [...event.dataTransfer.files]
.filter(file => file.type.startsWith("image/"));
// Pass droppedFiles to the same function used by the input handler.
renderImages(droppedFiles);
});Make the drop area keyboard-accessible by keeping a real file input inside it or next to it; drag-and-drop should enhance, not replace, the labeled picker. MDN’s file API guide includes drag-and-drop thumbnail handling and URL cleanup.
Object URLs versus FileReader
For a straightforward preview, object URLs are the direct pattern because the URL references the file rather than embedding its bytes into a string. FileReader.readAsDataURL() creates a data URL, which can be convenient when you explicitly need serialized text, but it increases the representation size and has data-URL size limitations. Object URLs still require explicit lifecycle cleanup.
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| Approach | Image source | Cleanup | Best fit |
|---|---|---|---|
URL.createObjectURL(file) |
Temporary blob URL referencing the local file | URL.revokeObjectURL(url) |
Normal previews and thumbnail galleries |
FileReader.readAsDataURL(file) |
Data URL containing an encoded representation | Discard the resulting string | Cases that specifically require inline serialized data |
Whichever method you use, never treat a client-generated preview as evidence that an upload is safe. Validate and process the original upload on the server.
Accessibility and user experience details
- Use a visible
<label>or an accessible name for every file input. - Give the preview meaningful alternative text, commonly the selected filename; if the image is decorative, use an empty
altvalue. - Expose rejection messages in an element with
role="alert"and avoid silently ignoring files. - Keep the preview hidden until a valid image exists, and clear the old image when a new selection is rejected.
- For galleries, announce changes with
aria-live="polite"and provide a clear/reset control. - Do not rely on color alone to show drag-over state.
Performance, privacy, and reliability considerations
Object URLs avoid an upload request and let the browser decode the local file. Large images can still consume memory while decoded, so cap client-side file size where a limit improves the experience and avoid creating previews for files you already know you will reject. For many files, revoke the complete URL list before rendering a replacement selection.
The selected bytes stay local until your code submits them. Do not log file contents or expose object URLs to unrelated page code. If you draw an image to a canvas, account for cross-origin rules and preserve the original file separately for upload.
Handle an image decode failure with an error listener so a damaged or unsupported file produces feedback:
preview.addEventListener("error", () => {
if (previewUrl) URL.revokeObjectURL(previewUrl);
previewUrl = undefined;
preview.removeAttribute("src");
preview.hidden = true;
error.textContent = "The selected file could not be displayed as an image.";
error.hidden = false;
});Keep the URL alive while the user may right-click, open, save, zoom, or otherwise interact with the preview. Revoke it when the component is removed; frameworks can call the cleanup from their unmount effect.
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Common problems and fixes
The image stays hidden
Confirm that the handler assigns preview.src and sets preview.hidden = false. Check the browser console for a selector typo and ensure the script runs after the elements exist.
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Look for URL.revokeObjectURL() in an immediate load callback. Move revocation to replacement, reset, or component teardown.
Nothing happens when the same file is selected again
After clearing the preview, set input.value = "". That resets the control so selecting the same path can generate another change event.
Some files are rejected
Inspect file.type and your size rule. Extensions are not reliable proof of content; retain server validation and explain the accepted formats in the UI.
Memory grows after repeated selections
Store every URL returned by createObjectURL() and revoke all old URLs before rendering a new set. Removing image elements alone is insufficient; see MDN’s revokeObjectURL() reference.
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Does previewing upload the file?
No. Creating an object URL only gives the current page a local reference. Upload occurs later when your code sends the original File.
Can I preview videos or PDFs this way?
The pattern is shown for images. Other media need an element and validation appropriate to that format.
Should I revoke a URL after the image loads?
Not if the preview remains interactive. Revoke it when replacing, clearing, or destroying the preview component.
Is accept="image/*" secure validation?
No. It guides selection. Validate file content and policy on the server before storing or processing the upload.
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