To preview a selected image before uploading it, handle the file input’s change event, read the first File from input.files, and set an <img> element’s src to URL.createObjectURL(file). Revoke the previous object URL when the user replaces or clears the image. If your application specifically needs the image represented as text, use FileReader.readAsDataURL() instead.
Basic image preview with an object URL
A browser cannot read an arbitrary local pathname supplied by script. The user must choose a file through a file input or provide it through a drag-and-drop operation. The resulting File object can be displayed without uploading it.
<label for="image-file">Choose an image</label>
<input id="image-file" type="file" accept="image/*">
<p id="image-status" role="status" aria-live="polite"></p>
<img id="image-preview" alt="Selected image preview" hidden>
<script>
const input = document.querySelector('#image-file');
const preview = document.querySelector('#image-preview');
const status = document.querySelector('#image-status');
let objectUrl = null;
input.addEventListener('change', () => {
const file = input.files?.[0];
if (!file) {
if (objectUrl) URL.revokeObjectURL(objectUrl);
objectUrl = null;
preview.removeAttribute('src');
preview.hidden = true;
status.textContent = 'No image selected.';
return;
}
if (!file.type.startsWith('image/')) {
if (objectUrl) URL.revokeObjectURL(objectUrl);
objectUrl = null;
preview.removeAttribute('src');
preview.hidden = true;
status.textContent = 'Please choose an image file.';
return;
}
if (objectUrl) URL.revokeObjectURL(objectUrl);
objectUrl = URL.createObjectURL(file);
preview.src = objectUrl;
preview.hidden = false;
status.textContent = `${file.name} selected.`;
});
window.addEventListener('beforeunload', () => {
if (objectUrl) URL.revokeObjectURL(objectUrl);
});
</script>accept="image/*" guides the file picker, but it is not complete validation. The type check in the example gives immediate feedback; validate uploads again in the service that receives them.
Why URL.createObjectURL() works
URL.createObjectURL(file) creates a temporary URL pointing at the selected File or another Blob. Assigning that URL to img.src lets the browser decode and display the image locally. Each call creates a unique URL, so retain the value and call URL.revokeObjectURL() after the preview is replaced, removed, or otherwise no longer needed. MDN documents this pattern in its file API examples.
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Using FileReader.readAsDataURL()
A data URL embeds the file content in a string. Choose this approach when another part of your application needs that data-URL representation, rather than merely a source for an image element.
<input id="data-file" type="file" accept="image/*">
<img id="data-preview" alt="Selected image preview" hidden>
<p id="data-status" role="status" aria-live="polite"></p>
<script>
const dataInput = document.querySelector('#data-file');
const dataPreview = document.querySelector('#data-preview');
const dataStatus = document.querySelector('#data-status');
dataInput.addEventListener('change', () => {
const file = dataInput.files?.[0];
if (!file) {
dataPreview.hidden = true;
dataStatus.textContent = 'No image selected.';
return;
}
if (!file.type.startsWith('image/')) {
dataPreview.hidden = true;
dataStatus.textContent = 'Please choose an image file.';
return;
}
const reader = new FileReader();
reader.addEventListener('load', () => {
dataPreview.src = reader.result;
dataPreview.hidden = false;
dataStatus.textContent = `${file.name} selected.`;
});
reader.addEventListener('error', () => {
dataPreview.hidden = true;
dataStatus.textContent = 'The image could not be read.';
});
reader.readAsDataURL(file);
});
</script>readAsDataURL() is asynchronous. Its result is available after the load event and can be assigned to src. See the MDN readAsDataURL reference and the broader FileReader documentation.
Object URL or data URL?
| Approach | Preview source | Cleanup | Use it when |
|---|---|---|---|
URL.createObjectURL(file) |
Temporary URL for a File or Blob |
Call URL.revokeObjectURL() when replaced or removed |
You only need to point an image element at the selected file |
FileReader.readAsDataURL(file) |
Data URL delivered through the reader’s result | No object URL to revoke | You specifically need the file content as a data URL |
The available documentation does not establish a universal performance winner. Select based on the representation your application needs and implement cleanup for dynamic object-URL previews.
Previewing several selected images
Add multiple, iterate through the FileList, and retain each URL so it can be revoked when its card is removed.
<input id="gallery-files" type="file" accept="image/*" multiple>
<div id="gallery"></div>
<p id="gallery-status" role="status" aria-live="polite"></p>
<script>
const galleryInput = document.querySelector('#gallery-files');
const gallery = document.querySelector('#gallery');
const galleryStatus = document.querySelector('#gallery-status');
const urls = new Set();
galleryInput.addEventListener('change', () => {
gallery.replaceChildren();
for (const url of urls) URL.revokeObjectURL(url);
urls.clear();
const files = [...(galleryInput.files || [])];
const images = files.filter(file => file.type.startsWith('image/'));
galleryStatus.textContent = images.length
? `${images.length} image${images.length === 1 ? '' : 's'} selected.`
: 'No supported images selected.';
for (const file of images) {
const url = URL.createObjectURL(file);
urls.add(url);
const figure = document.createElement('figure');
const image = document.createElement('img');
image.src = url;
image.alt = `Preview of ${file.name}`;
const caption = document.createElement('figcaption');
caption.textContent = file.name;
figure.append(image, caption);
gallery.append(figure);
}
});
window.addEventListener('beforeunload', () => {
for (const url of urls) URL.revokeObjectURL(url);
});
</script>For a production gallery, associate each URL with its preview element and revoke it immediately when that individual preview is deleted. The same routine can process files supplied by drag and drop; MDN describes this shared flow in its file drag-and-drop guide.
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Adding drag and drop without removing the picker
Keep a real, labeled input available for keyboard and assistive-technology users. A drop zone can forward its files to the same preview function:
<label id="drop-zone" for="drop-input">Drop images here or choose files</label>
<input id="drop-input" type="file" accept="image/*" multiple>
<div id="drop-gallery"></div>
<script>
const zone = document.querySelector('#drop-zone');
const dropInput = document.querySelector('#drop-input');
function previewFiles(fileList) {
// Call the same gallery-rendering routine used by the input change handler.
// fileList is a FileList or an array of File objects.
}
dropInput.addEventListener('change', event => previewFiles(event.target.files));
zone.addEventListener('dragover', event => {
event.preventDefault();
zone.dataset.dragging = 'true';
});
zone.addEventListener('dragleave', () => delete zone.dataset.dragging);
zone.addEventListener('drop', event => {
event.preventDefault();
delete zone.dataset.dragging;
previewFiles(event.dataTransfer.files);
});
</script>In a complete implementation, put the gallery code from the multiple-file example inside previewFiles. Announce unsupported files and empty drops in visible text or a live status region rather than relying only on color or a temporary visual effect.
Accessibility and interface behavior
- Use a visible label or a label-backed control with a meaningful name.
- Give each preview useful alternative text; for a purely decorative duplicate, use an empty
altvalue instead. - Provide a text status for selection errors, unsupported files, and an empty selection.
- Explain how to replace or clear the current preview.
- Do not hide the only file-picker control when adding drag and drop.
These practices make the interaction understandable even when a drop zone, image decoding, or CSS styling is unavailable.
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The preview stays blank
Confirm that the handler reads input.files[0], that a file was actually selected, and that preview.src receives the object URL or completed reader result. Check the browser console for errors and ensure the image is not still hidden.
The old image remains after choosing a new one
Revoke the previous URL before assigning the new one, clear the old src when there is no selection, and reset the preview’s visibility.
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Memory grows after repeated replacements
Object URLs remain usable until revoked or the document is unloaded. Store the current URL, revoke it on replacement and removal, and revoke all URLs in a multi-image collection when the collection is cleared.
accept="image/*" did not prevent a non-image file
accept is a picker hint, not a complete validation mechanism. Inspect the selected file’s type for immediate UI feedback and perform authoritative validation when processing the upload.
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Cancel the dragover and drop events with preventDefault(). Without that, the browser may handle the drop as navigation instead of exposing dataTransfer.files. Ensure the drop target is large enough and that the picker remains usable.
The data-URL example has no result yet
FileReader completes asynchronously. Set src inside its load handler, not immediately after calling readAsDataURL().
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Further references
For file-selection behavior, see MDN’s file input reference and File API overview. Browser compatibility and documentation can change, so verify support for any specific browser/version matrix you must guarantee.
Frequently Asked Questions
Can JavaScript read a user’s local file path?
No. The browser exposes a user-selected File through a file input or drop operation, not an arbitrary pathname.
Do I need to upload the image to preview it?
No. An object URL or FileReader data URL can display the selected file locally before any network request.
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Revoke it when its preview is replaced, removed, or cleared; also revoke retained URLs when a dynamic gallery is destroyed.
Can the same code handle camera captures?
If the browser supplies the capture as a File through the input, the same preview handler can process it.
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