An empty or null PDF download URL usually means a value failed somewhere in the pipeline—not that the PDF format itself is broken. The request URL may be missing, fetch() may have returned an HTTP error or an unreadable opaque response, the response may not have been converted to a Blob, or the link may be assigned before an asynchronous operation finishes. Trace the values in that order: endpoint, response, body, Blob, object URL, and finally the anchor.
Find the stage that becomes empty
Start by logging the endpoint variable before making the request and the link’s href immediately before the user clicks it:
console.log('endpoint:', endpoint);
const response = await fetch(endpoint);
console.log('status:', response.status, 'ok:', response.ok, 'final URL:', response.url);
// ...after creating the URL...
console.log('href:', link.href);If endpoint is already null, inspect the application state, route parameter, form value, or API response that should supply it. If the endpoint is valid but href is empty, inspect the awaited promise, exception handler, and the code that assigns the link. A common mistake is returning from an asynchronous function before the Blob URL has been created.
Do not treat a fulfilled fetch() promise as proof of success. Fetch normally fulfills with a Response even for statuses such as 404 or 500. Check response.ok (or the numeric status) before reading the body.
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Use the correct bytes-to-link flow
When your endpoint returns PDF bytes to browser JavaScript, the reliable sequence is:
- Call
fetch()with a real endpoint. - Reject non-success HTTP statuses.
- Confirm that the response is the expected kind of content.
- Read the body with
response.blob(). - Verify that the Blob is not empty.
- Create a temporary URL with
URL.createObjectURL(blob). - Assign that string to the anchor’s
hrefbefore the click.
A Blob contains bytes and metadata; it is not itself a URL. URL.createObjectURL() is the step that produces a browser-local blob: address.
Complete browser example
async function preparePdfDownload(endpoint, link) {
if (!endpoint) {
throw new Error('No PDF endpoint was supplied');
}
const response = await fetch(endpoint, {
credentials: 'include' // keep only if your endpoint uses browser cookies
});
if (!response.ok) {
throw new Error(`PDF request failed: ${response.status} ${response.statusText}`);
}
const contentType = response.headers.get('content-type');
if (!contentType || !contentType.toLowerCase().includes('application/pdf')) {
throw new TypeError(`Expected application/pdf, got ${contentType || 'no content type'}`);
}
const blob = await response.blob();
if (blob.size === 0) {
throw new Error('PDF response body is empty');
}
const objectUrl = URL.createObjectURL(blob);
link.href = objectUrl;
link.download = 'document.pdf';
// Revoke objectUrl after the download has been initiated and the browser
// no longer needs it. Do not revoke it before the click consumes it.
link.addEventListener('click', () => {
setTimeout(() => URL.revokeObjectURL(objectUrl), 1000);
}, { once: true });
}Call it from a user action and handle errors visibly:
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downloadButton.addEventListener('click', async () => {
downloadButton.disabled = true;
try {
await preparePdfDownload('/api/invoices/42/pdf', pdfLink);
pdfLink.click();
} catch (error) {
console.error(error);
status.textContent = error.message;
} finally {
downloadButton.disabled = false;
}
});The content-type and zero-size checks are practical safeguards. A server can return an HTML login page or JSON error with a successful transport, so also inspect the actual response when the file still does not open. A content type alone cannot prove that the bytes are a valid PDF.
Opaque responses and CORS: why the body appears to be null
An opaque response is what the browser exposes when a cross-origin request is made without readable CORS permission, commonly after using mode: "no-cors". It has status 0, no exposed headers, and a null body. Calling blob() on it produces a zero-size Blob with an empty type, which cannot provide a useful PDF download.
Do not use no-cors as a workaround. Configure the PDF server to return an appropriate Access-Control-Allow-Origin value for the requesting origin (and any credentials headers required by your authentication model), then make a normal request. If you cannot change the remote server, fetch the file through your own authenticated backend instead of trying to read it directly in the browser.
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What to inspect in DevTools
- Request URL: confirm that it is the intended endpoint, not an undefined route or a page URL.
- Status: investigate 3xx redirects, 401/403 authentication failures, 404 routes, and 5xx server errors.
- Final URL:
response.urlreports the URL after redirects and can reveal a login page or a different host. - Response headers: check
Content-Typeand, for direct downloads,Content-Disposition. - Preview or response body: look for an HTML error page, JSON error object, or empty body instead of PDF bytes.
Choose between a direct URL and a Blob URL
| Flow | Use it when | Checks and trade-offs |
|---|---|---|
| Direct server URL | Your server returns a stable, downloadable address. | Verify redirects, origin, authentication, media type, and disposition headers. The server can provide a suggested filename. The browser controls whether the file is saved or opened. |
| Fetched bytes plus Blob URL | The PDF is protected, must be inspected, or must be transformed before the user receives it. | Requires readable CORS or a same-origin request. The complete response is read into browser memory before the Blob URL is created. Revoke the object URL after use. |
For a direct link, do not manufacture a blob: URL unless you have actually fetched bytes. Use the URL returned by your API, and inspect the final response when redirects are involved.
Why the anchor still does not download
The download attribute has limits
The anchor’s download attribute applies to same-origin URLs and to blob: or data: URLs. It does not force a cross-origin URL to download. Browser settings and server headers can also cause the document to open in a tab instead of being saved.
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Server headers can override expectations
Content-Disposition influences whether a response is treated as an attachment and can supply a suggested filename. The response media type also affects handling. Check both headers on the final response, especially after redirects.
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Click timing and cleanup
Assign href before calling click(). Do not revoke the object URL immediately after assigning it; revoke it only after the browser has had an opportunity to consume the download. If a framework re-renders the anchor, verify that it did not replace your assigned attribute.
Common failure patterns and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
endpoint logs as null or undefined |
Missing state, route parameter, or API field. | Validate the input before fetch(); trace the code that populates it and handle an absent value explicitly. |
| Fetch resolves with 404 or 500 | HTTP errors do not automatically reject fetch(). |
Check response.ok and stop before creating a Blob. |
| Status is 0, headers are empty, Blob size is 0 | Opaque cross-origin response. | Remove no-cors; configure CORS or proxy the request through your server. |
| Blob URL exists but file is an HTML page | Authentication redirect, login page, proxy error, or wrong endpoint. | Inspect response.url, content type, and response bytes; send the required credentials. |
| Link opens but does not save | Cross-origin URL, browser policy, or disposition/header behavior. | Use a same-origin or Blob URL for download, and inspect Content-Disposition. |
| Download works intermittently | Race between asynchronous fetch and click, or premature URL revocation. | await every step, disable the button while loading, and revoke only after the click. |
Performance, authentication, and reliability considerations
- Large PDFs: Blob creation reads the complete response, so memory use rises with document size. A direct server download can avoid duplicating those bytes in the page.
- Protected files: Decide whether cookies, an authorization header, or a backend proxy should supply credentials. Never expose a long-lived private token in a public link.
- Retries: Retry only transient network or server failures, not a deterministic 401, 403, or 404. Show the status to the user and log a request identifier if your server provides one.
- Validation: Treat an unexpected content type or empty body as an application error, not as a valid download.
- Object URL lifecycle: Keep the URL alive through the click or open operation, then revoke it to release browser resources.
Or skip the browser setup
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Use the documented API parameters and authentication shown in the ScreenshotNeo documentation:
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curl -G "https://api.screenshotneo.com/v1/shot"
-d access_key=YOUR_API_KEY
--data-urlencode url=https://stripe.com
-o shot.pdfimport requests
r = requests.get(
"https://api.screenshotneo.com/v1/shot",
params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"},
timeout=90,
)
r.raise_for_status()
open("shot.pdf", "wb").write(r.content)const q = new URLSearchParams({
access_key: 'YOUR_API_KEY',
url: 'https://stripe.com'
});
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);
if (!res.ok) throw new Error(`ScreenshotNeo failed: ${res.status}`);
const bytes = Buffer.from(await res.arrayBuffer());
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Short diagnostic checklist
- Log the original endpoint and reject a missing value.
- Inspect status,
response.ok, andresponse.url. - Confirm CORS is readable; never rely on
no-corsfor PDF bytes. - Verify the content type and nonzero body.
- Await
response.blob(), then callURL.createObjectURL(). - Assign
hrefbefore clicking and revoke the URL afterward. - For direct links, inspect origin, redirects,
Content-Disposition, and browser download behavior.
Frequently Asked Questions
Can I turn a PDF Blob directly into a download URL?
No. A Blob holds the response bytes. Call URL.createObjectURL(blob) to obtain a temporary blob: URL, then assign that string to the link.
Should I use data URLs instead of Blob URLs?
A data URL can work for small payloads, but it encodes the entire file into the URL. Blob URLs are the usual choice for fetched PDF responses and still require eventual revocation.
What information is needed to diagnose one specific null URL?
Provide the code that supplies the endpoint and assigns href, the request status and final URL, response headers, and whether the response is opaque. Those details identify which pipeline stage failed.
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