For a client-side download created with URL.createObjectURL(), intercept the Blob in the page—not the network request. Install a hook before the application runs, trigger its download action, then read the Blob in the page context and transfer its bytes to Node.js. This works because a blob: URL points to data held by the browser rather than a conventional file response.
If the page already exposes a normal <a download> link and you only need a file on disk, you can instead let Chrome save it. The right approach depends on whether you need to inspect or transform the bytes, or simply preserve the browser’s download behavior.
Why Puppeteer request interception misses a Blob download
A page can generate data locally, wrap it in a Blob, and call URL.createObjectURL(blob). The resulting blob: URL is an opaque identifier for that browser-managed object. It is not the same as a server URL returning a file, so enabling Puppeteer request interception does not give you a reliable way to capture the Blob’s bytes as a normal network response.
The useful control point is page JavaScript. You can either obtain the Blob URL and fetch it from the page that created it, or instrument URL.createObjectURL() before the application calls it. Puppeteer’s page.evaluate() executes in the page context and can await a Promise, making it suitable for reading the Blob and returning its bytes to Node.js.
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Blob URLs can be revoked with URL.revokeObjectURL(). Read or retain the Blob before application code revokes the URL; revoking too early can make the URL unusable, while never releasing URLs can retain the underlying object.
Choose the interception method
| Approach | Use it when | Main trade-off |
|---|---|---|
Read the Blob through page.evaluate() |
You have a Blob URL and need the bytes for parsing, hashing, or a custom filename. | Bytes cross the page-to-Node boundary and may consume substantial memory. |
Instrument URL.createObjectURL() |
The application creates the Blob URL internally and does not expose it through an accessible selector or API. | The hook must run before URL creation; account for frames and release retained Blob references after use. |
| Let Chrome download it | The page has an anchor download flow and you need the browser to write a file without transforming it. | Chrome determines download behavior and filename; you must configure a writable destination. |
Extract Blob bytes in Node.js
This complete example records Blobs as the application creates their object URLs, triggers a configurable page control, reads the latest recorded Blob in its creating page, and writes its bytes to disk. Change targetUrl, downloadControl, and outputPath for the site you are automating. Install Puppeteer in your project before running it.
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const puppeteer = require('puppeteer');
const fs = require('node:fs/promises');
const path = require('node:path');
async function main() {
const targetUrl = 'https://example.com';
const downloadControl = 'button.export';
const outputPath = path.resolve('download.bin');
const browser = await puppeteer.launch({ headless: true });
try {
const page = await browser.newPage();
// Install before page code runs. Keep the original behavior intact,
// but retain Blob objects so a quick URL revocation cannot erase them.
await page.evaluateOnNewDocument(() => {
const original = URL.createObjectURL.bind(URL);
window.__blobRecords = [];
URL.createObjectURL = (value) => {
const url = original(value);
window.__blobRecords.push({
url,
blob: value instanceof Blob ? value : null,
type: value instanceof Blob ? value.type : ''
});
return url;
};
});
await page.goto(targetUrl, { waitUntil: 'domcontentloaded' });
await page.click(downloadControl);
const result = await page.evaluate(async () => {
const records = window.__blobRecords || [];
const record = records[records.length - 1];
if (!record) throw new Error('No object URL was recorded');
const blob = record.blob || await (async () => {
const response = await fetch(record.url);
if (!response.ok) throw new Error(`Blob fetch failed: ${response.status}`);
return response.blob();
})();
const bytes = await blob.arrayBuffer();
return {
url: record.url,
type: blob.type || record.type,
bytes: Array.from(new Uint8Array(bytes))
};
});
await fs.mkdir(path.dirname(outputPath), { recursive: true });
await fs.writeFile(outputPath, Buffer.from(result.bytes));
console.log(`Saved ${result.bytes.length} bytes (${result.type || 'unknown type'}) to ${outputPath}`);
} finally {
await browser.close();
}
}
main().catch((error) => {
console.error(error);
process.exitCode = 1;
});The wrapper calls the original createObjectURL() and returns its URL unchanged. It retains a Blob reference when the argument is a Blob, so the code can read that object even if the application quickly revokes its URL. If the application creates the URL in a different frame, inspect and instrument that frame too; a main-frame hook will not capture creation performed elsewhere.
The code writes bytes, not a semantically named document. Choose an extension based on the application’s expected file format or inspect result.type; MIME type may help identify the format, but do not assume every Blob has one. For large files, avoid keeping multiple copies of the data alive longer than necessary: this example holds the serialized byte array in Node while writing it.
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Capture only the Blob URL when that is enough
If the application does not revoke the URL immediately and you do not need to preserve a Blob reference, a smaller hook can record just the URL. Install it before navigation or before the application bundle executes:
await page.evaluateOnNewDocument(() => {
const original = URL.createObjectURL.bind(URL);
window.__blobUrls = [];
URL.createObjectURL = (value) => {
const url = original(value);
window.__blobUrls.push(url);
return url;
};
});After navigating and performing the UI action, get the recorded URLs with await page.evaluate(() => window.__blobUrls || []). Pass the selected URL to page.evaluate() and use fetch(blobUrl), response.blob(), and blob.arrayBuffer() to read it. Fetch it in the page context that created it: moving a Blob URL into an unrelated page or browser context can fail because its access is tied to the creating context.
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A recorded URL is only useful while it remains valid. If the application revokes it before your follow-up evaluation, capture and retain the Blob itself in the hook, as in the full example, or arrange to read it before revocation.
Let Chrome save an anchor download
When the page already has a conventional <a download> flow and you do not need to inspect the bytes, configuring Chrome’s download policy is simpler. The following launch configuration allows downloads to an absolute directory:
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const path = require('node:path');
const puppeteer = require('puppeteer');
const browser = await puppeteer.launch({
downloadBehavior: {
policy: 'allow',
downloadPath: path.resolve('downloads')
}
});
const page = await browser.newPage();
await page.goto('https://example.com');
await page.click('a[download]');For lower-level control, attach a Chrome DevTools Protocol session to the page and call Browser.setDownloadBehavior with behavior: 'allow' or behavior: 'allowAndName' and an absolute downloadPath. The page session is created with await page.createCDPSession(). If your installed Puppeteer version does not accept the launch option, use the CDP method supported by that version rather than assuming the launch configuration is available.
Starting a click is not the same as verifying a completed file. Make your automation check that the expected output exists and is complete before consuming it; for variable download times, use a bounded wait and report a timeout instead of reading a partial file. Set the policy and writable directory before clicking so Chrome does not deny the download.
The HTML download attribute supports blob: and data: URLs, but filename selection can also be affected by Content-Disposition, URL path, and Blob.type. If a particular filename matters, inspect the link’s download value and verify the saved result in your workflow.
Common failures and how to fix them
- No URL was captured: Install the hook before navigation or before the application code runs, then trigger the action that actually generates the file. Check relevant frames if the download control runs inside an iframe.
- The URL is present but fetching fails: Fetch it in the page context that created it and check whether the application revoked it. Retaining the Blob in the hook avoids relying on the URL remaining live.
- The output is empty or truncated: Await
response.blob()andblob.arrayBuffer()before writing, and do not revoke the URL before extraction. In the browser-managed path, wait for the file to finish before opening it. - Chrome denies the download: Set the browser or context download policy before clicking, and make sure the destination is an absolute writable path.
- The filename is unexpected: Check the anchor’s
downloadattribute, the Blob MIME type, and any server-providedContent-Dispositionvalue. Browser naming rules can make the saved name differ from the one you expected. - Memory use grows: Do not keep a long-lived array of large Blobs or byte arrays. Release your instrumentation records after extracting what you need, and let the page release object URLs when their lifecycle permits.
Or skip the browser setup
ScreenshotNeo is a website screenshot API, not a Blob-download interceptor: it will not extract a page’s client-side file. It is relevant if what you actually need is a screenshot or PDF of a page, rather than the downloaded bytes. A single request looks like this; see the ScreenshotNeo API documentation for its request options.
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Which method should you use?
Use page-side extraction when you need deterministic bytes, custom naming, parsing, or hashing. Instrument createObjectURL() when the application hides the Blob URL, and retain the Blob if revocation may happen quickly. Use Chrome’s download policy when a regular anchor download is sufficient and browser-managed filenames are acceptable. These approaches address different parts of the workflow: request interception is not the right mechanism for data created locally in a page.
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