Preview a selected image in Angular by handling the file input’s change event, keeping the chosen File, and assigning URL.createObjectURL(file) to an image element. Revoke that object URL when the preview is replaced, cleared, or the component is destroyed. The preview is local browser work; upload the original file separately when the user submits the form.
Minimal Angular preview
This standalone pattern works with a template-driven or otherwise ordinary Angular component. The accept attribute narrows the picker to images, but it is only a user-interface hint. Your component must still validate the selected file.
Template
<input
type="file"
accept="image/*"
(change)="onImageSelected($event)"
>
<p *ngIf="validationError" role="alert">
{{ validationError }}
</p>
<img
*ngIf="previewUrl"
[src]="previewUrl"
alt="Selected image preview"
>
<button type="button" (click)="clearPreview()" [disabled]="!previewUrl">
Clear
</button>
<button type="button" (click)="upload()" [disabled]="!selectedFile">
Upload
</button>Component code
import { Component, OnDestroy } from '@angular/core';
@Component({
selector: 'app-image-picker',
templateUrl: './image-picker.component.html'
})
export class ImagePickerComponent implements OnDestroy {
previewUrl: string | null = null;
selectedFile: File | null = null;
validationError = '';
onImageSelected(event: Event): void {
const input = event.target as HTMLInputElement;
const file = input.files?.[0] ?? null;
this.clearPreview();
this.validationError = '';
if (!file) {
return;
}
if (!file.type.startsWith('image/')) {
this.validationError = 'Choose an image file.';
return;
}
this.selectedFile = file;
this.previewUrl = URL.createObjectURL(file);
}
clearPreview(): void {
if (this.previewUrl) {
URL.revokeObjectURL(this.previewUrl);
}
this.previewUrl = null;
this.selectedFile = null;
}
upload(): void {
if (!this.selectedFile) {
return;
}
// Send this.selectedFile with your application’s upload request.
}
ngOnDestroy(): void {
this.clearPreview();
}
}URL.createObjectURL() returns a temporary URL representing the selected file. Angular binds it to src, while selectedFile remains available for the eventual upload. Do not revoke the URL immediately after assigning it: the image still needs it. Revoke it when it is no longer displayed.
What happens when the user selects a file
- The browser fires
changeon the file input. - The handler reads the first entry from
input.files. A web page can access files the user selected (or dropped), not arbitrary paths on the user’s computer. - The component checks the file’s type and any application-specific limits.
- An object URL is generated and stored in component state.
- Angular updates the image’s
src, so the browser renders the local file without a network upload. - When the selection changes, the old URL is revoked before the new one is stored.
Previewing does not send data to your server. Keep the File object and perform an explicit upload, commonly with FormData, when the user confirms.
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Validation that belongs in a real component
Type and extension
accept="image/*" improves the picker experience, but it does not establish that a file is safe or acceptable. Check file.type, and if your policy allows only particular formats, use an explicit allow-list such as image/png, image/jpeg, or image/webp. File names and extensions are user-controlled.
Size limits
Reject files above the limit your application can process before creating a preview. For example:
const maxBytes = 5 * 1024 * 1024; // 5 MiB
if (file.size > maxBytes) {
this.validationError = 'The image must be 5 MiB or smaller.';
return;
}Choose a limit that matches your server and image-processing pipeline; the number above is only an example.
Dimensions
If your UI requires a minimum or maximum width and height, inspect the object URL with an Image before accepting it:
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private validateDimensions(file: File, url: string): Promise<void> {
return new Promise((resolve, reject) => {
const image = new Image();
image.onload = () => {
if (image.width < 400 || image.height < 300) {
reject(new Error('Image must be at least 400 × 300 pixels.'));
} else {
resolve();
}
};
image.onerror = () => reject(new Error('The image could not be decoded.'));
image.src = url;
});
}Whichever checks you add in the browser, repeat authoritative validation on the server. Client-side validation is feedback, not a security boundary.
Replacing and clearing previews safely
Every generated object URL should have a matching URL.revokeObjectURL(). The example’s clearPreview() handles both replacement and manual clearing, and ngOnDestroy() covers navigation away from the component. Clearing selectedFile prevents an old file from being uploaded after its preview has disappeared.
If you reset the native input, also clear its value so selecting the same file again fires change:
clearInput(input: HTMLInputElement): void {
this.clearPreview();
input.value = '';
}Call this method from a clear button by passing a template reference, or use @ViewChild if that better fits your form.
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Object URL or FileReader?
| Approach | Result | Best fit | Cleanup |
|---|---|---|---|
URL.createObjectURL(file) |
A temporary URL referring to the selected File |
A straightforward, display-only preview | Revoke when replaced, removed, or destroyed |
FileReader.readAsDataURL(file) |
An asynchronously produced data: URL containing encoded file data |
Code that specifically needs a data URL string | Discard the reader result when the preview is replaced; there is no object URL to revoke |
FileReader example
onImageSelectedWithReader(event: Event): void {
const input = event.target as HTMLInputElement;
const file = input.files?.[0] ?? null;
this.clearPreview();
if (!file || !file.type.startsWith('image/')) {
this.validationError = 'Choose an image file.';
return;
}
this.selectedFile = file;
const reader = new FileReader();
reader.addEventListener('load', () => {
this.previewUrl = typeof reader.result === 'string'
? reader.result
: null;
});
reader.addEventListener('error', () => {
this.validationError = 'The image could not be read.';
this.selectedFile = null;
});
reader.readAsDataURL(file);
}A data URL can be convenient when another API explicitly requires one, but it represents the file as encoded text. For simply setting an image source, an object URL is the more direct choice.
Multiple-image previews
Use multiple, iterate through all selected files, and associate each URL with its file. Revoke the individual URL when a thumbnail is removed and all remaining URLs when the component is destroyed.
previews: Array<{ file: File; url: string }> = [];
onImagesSelected(event: Event): void {
const input = event.target as HTMLInputElement;
this.clearAllPreviews();
for (const file of Array.from(input.files ?? [])) {
if (file.type.startsWith('image/')) {
this.previews.push({ file, url: URL.createObjectURL(file) });
}
}
}
removePreview(index: number): void {
const [removed] = this.previews.splice(index, 1);
if (removed) URL.revokeObjectURL(removed.url);
}
clearAllPreviews(): void {
for (const preview of this.previews) {
URL.revokeObjectURL(preview.url);
}
this.previews = [];
}
ngOnDestroy(): void {
this.clearAllPreviews();
}Uploading after the preview
Keep upload logic separate so a user can inspect, replace, or cancel a selection. A typical Angular service call sends the original file:
const form = new FormData();
form.append('image', this.selectedFile, this.selectedFile.name);
this.http.post('/api/images', form).subscribe({
next: () => this.clearPreview(),
error: () => this.validationError = 'Upload failed. Try again.'
});Do not manually set the multipart Content-Type header when using FormData; the browser supplies the boundary. Your server should verify the decoded content, size, dimensions, and authorization independently of browser checks.
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Troubleshooting
The preview is blank
- Confirm that
previewUrlis assigned and that the template uses[src]="previewUrl", not an interpolated literal with a stale value. - Check that the selected file is an image and that the browser can decode it.
- Make sure you did not call
URL.revokeObjectURL()immediately after assigning the URL.
Selecting the same file does nothing
File inputs may not emit change when their value has not changed. Set the input’s value to an empty string after clearing, as shown above.
The old image remains after choosing a new one
Clear the previous URL and file at the start of the handler. Do not append a new preview without removing the old state.
Memory usage grows after many selections
Each object URL holds a reference until revoked. Revoke on replacement, removal, and component destruction; for multiple files, track and revoke every URL.
A valid-looking file is rejected
Some browsers provide an empty or unexpected MIME type. If your product permits that case, combine a cautious extension check with server-side content verification rather than trusting either value alone.
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The upload still succeeds after clearing
Clear both previewUrl and selectedFile. The image element disappearing does not automatically remove the file stored in component state.
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Can Angular preview an image without uploading it?
Yes. A user-selected File can be rendered locally with an object URL or a FileReader data URL; no request is made until your upload code sends the File.
Should I store the preview URL in a database?
No. An object URL is local, temporary browser state. Store the uploaded asset’s server URL only after your server accepts and stores the file.
Why does the file input expose a File object instead of a local path?
Browser security intentionally gives scripts the user-authorized File data while withholding unrestricted access to arbitrary local filesystem paths.
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