To make Puppeteer finish each PDF before the loop advances, use an async function, a for...of loop, and await on both navigation and page.pdf(). Puppeteer’s page.pdf() returns a promise for PDF bytes, so starting it without awaiting or returning that promise can let later work—such as merging files—run too soon. The exact cause of skipped output depends on your loop and helper code.
Generate PDFs one at a time with an awaited loop
Use this pattern when order matters, jobs depend on one another, or you want to limit simultaneous browser work. It waits for navigation and PDF generation in each iteration before starting the next.
async function generatePdfs(items, browser) {
const results = [];
for (const [index, item] of items.entries()) {
const page = await browser.newPage();
try {
await page.goto(item.url, { waitUntil: 'networkidle2' });
const pdf = await page.pdf({ format: 'A4' });
results.push({ index, pdf });
} finally {
await page.close();
}
}
return results;
}The await on page.pdf() is the key: Puppeteer documents this method as returning a Promise<Uint8Array>, meaning the PDF bytes arrive asynchronously. The Puppeteer PDF guide demonstrates printing a page with Page.pdf(); the API reference documents its return value and notes that print CSS is the default.
If you write files rather than keeping buffers, assign a different path to each iteration, such as report-${index}.pdf. Await the file-writing operation too. Reusing one path for concurrent writes can overwrite output and make it appear that iterations were skipped.
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Make sure the caller waits for the helper
Awaiting inside the loop only works as intended when the surrounding function is awaited by its caller. A helper can return the promise from page.pdf(), or await it before returning; if it launches asynchronous work but neither awaits nor returns that promise, its caller may finish first.
async function makePdf(page) {
return page.pdf({ format: 'A4' });
}
async function run(items, browser) {
for (const item of items) {
const page = await browser.newPage();
try {
await page.goto(item.url, { waitUntil: 'networkidle2' });
const pdf = await makePdf(page);
// Consume or save pdf here before advancing.
} finally {
await page.close();
}
}
}Callback-based iteration is another common source of confusion: a loop construct does not automatically wait for asynchronous callbacks. If you use callbacks, confirm that each callback returns its promise and that the surrounding code waits for those promises. For straightforward sequential work, for...of makes the waiting relationship explicit.
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Use concurrency only when the jobs are independent
Promise.all waits for a group of promises, but it starts those jobs concurrently rather than one at a time. Use it when each PDF can be generated independently and your environment has capacity for multiple pages.
const pdfBuffers = await Promise.all(
items.map(async (item) => {
const page = await browser.newPage();
try {
await page.goto(item.url, { waitUntil: 'networkidle2' });
return await page.pdf({ format: 'A4' });
} finally {
await page.close();
}
}),
);For a large input set, avoid launching every page at once. Process batches or apply a concurrency limit appropriate to the memory, CPU, and browser capacity available to your application. Sequential work is simpler to reason about and bounds simultaneous browser activity; concurrency may improve throughput when the available resources permit it, but there is no universal speed gain.
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| Choose | When it fits | What to watch |
|---|---|---|
Sequential for...of with await |
Order matters, results depend on earlier jobs, or you need to constrain simultaneous browser work. | Total work proceeds one item at a time; handle each failure and close each page. |
Promise.all or bounded concurrency |
Jobs are independent and the browser host can support parallel pages. | Unbounded parallel pages can strain resources; use unique output paths and decide how failures should affect the group. |
Diagnose missing or apparently skipped PDFs
Without the loop, helper, error output, Puppeteer and Node.js versions, and runtime details, it is not possible to identify one guaranteed cause. Check these likely failure points in order:
- The PDF promise is not awaited or returned. Await
page.pdf()in the helper or return its promise, then await the helper at the call site. - The iteration callback’s promise is dropped. Prefer an
async for...ofloop for serial work, or explicitly collect and await promises for parallel work. - Merging starts before generation finishes. Await the complete sequential function or the full promise group before passing files to the merger.
- An error interrupts later work. Inspect the error output and decide whether a failed item should stop the batch or be recorded while later items continue. Catch errors at the appropriate per-item or batch boundary rather than silently discarding them.
- Outputs collide. Give each item a unique filename and confirm that your save operation has completed before using the file.
- A page is not cleaned up on failure. Put
page.close()in afinallyblock so failed navigation or PDF generation does not leave that page open.
Or skip the browser setup
If your goal is a screenshot rather than a PDF, ScreenshotNeo provides a one-request screenshot API. This example follows the ScreenshotNeo documentation:
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Does page.pdf() return a file path?
No. Puppeteer documents it as returning a promise for PDF bytes (Uint8Array). Save those bytes separately if you need a file.
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Does Promise.all make PDF generation sequential?
No. It waits for all supplied promises while their work runs concurrently; use an awaited for...of loop for one-at-a-time processing.
Can I merge PDFs after the loop?
Yes. Wait for the function that generates all buffers or files to finish before passing its completed outputs to your PDF-merging step.
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