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Use customElements.whenDefined() to wait until the browser knows the custom element, then wait for the page’s own signal that its data and rendering are ready before calling chromedp’s PDF command. A defined element is not necessarily finished rendering, and neither a selector wait nor network idleness proves that it is.
What “ready” means for a custom element
There are two separate milestones: the browser registers the element’s constructor, and the component finishes the work needed for the printed document. The HTML Standard defines customElements.whenDefined(name) as a promise fulfilled with the constructor when that valid custom element becomes defined; if it is already defined, the promise fulfills immediately. See the WHATWG HTML Standard.
That promise does not wait for a component’s network requests, data fetching, nested components, chart drawing, image loading, or layout to settle. There is no universal browser property named ready that covers these tasks. Use a readiness contract documented by the page or component, or have the page expose one explicitly.
Wait for definition and page readiness with chromedp
The example below navigates to a page, waits for the report-card definition, checks that an instance exists, awaits a page-owned readiness promise, waits for document fonts, and then writes PDF bytes to a file. The readiness property is deliberately application-specific: window.__PDF_READY__ is an example, not a built-in browser API.
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package main
import (
"context"
"os"
"time"
"github.com/chromedp/cdproto/page"
"github.com/chromedp/chromedp"
)
func capturePDF(parent context.Context, targetURL, outputPath string) error {
// Bound the navigation, readiness wait, and PDF generation together.
ctx, cancel := context.WithTimeout(parent, 60*time.Second)
defer cancel()
// NewContext controls a compatible Chrome/Chromium instance. If your
// application uses a remote browser, create the chromedp context for it.
ctx, cancelBrowser := chromedp.NewContext(ctx)
defer cancelBrowser()
var pdf []byte
err := chromedp.Run(ctx,
chromedp.Navigate(targetURL),
chromedp.Evaluate(`(async () => {
await customElements.whenDefined('report-card');
const el = document.querySelector('report-card');
if (!el) throw new Error('report-card was not found');
if (!window.__PDF_READY__) {
throw new Error('page must expose its PDF readiness promise');
}
await window.__PDF_READY__;
await document.fonts.ready;
return true;
})()`, nil),
chromedp.ActionFunc(func(ctx context.Context) error {
var err error
pdf, _, err = page.PrintToPDF().WithPrintBackground(true).Do(ctx)
return err
}),
)
if err != nil {
return err
}
return os.WriteFile(outputPath, pdf, 0o644)
}
This is an implementation pattern, not a tested program for every chromedp release or target page. Pin compatible versions of chromedp and cdproto, and verify their APIs. The code uses page.PrintToPDF and the WithPrintBackground option; the generated Page API returns PDF bytes, which the example saves. Consult the cdproto Page API and the chromedp project documentation for version-specific details.
Expose a readiness contract on the page
The page should resolve its PDF readiness promise only after the content required in the printed output is complete. For example, application code might assign a promise that encompasses data loading and component rendering:
window.__PDF_READY__ = (async () => {
await loadReportData();
await renderReportCards();
await waitForRequiredImages();
})();The function names above are illustrative; use the target application’s real operations. Reject the promise when an unrecoverable failure means the document cannot be prepared. In production, make failures observable in Go and return them rather than printing a partial document.
If the component already documents a readiness promise or state, wait for that contract instead of adding a second one. If several tag definitions matter, await them together:
await Promise.all([
customElements.whenDefined('report-card'),
customElements.whenDefined('report-chart'),
]);Then wait for whatever application state establishes that those components have rendered the required content. If the relevant element is inside another frame, run the wait in that frame’s execution context. A top-level registry lookup or selector is not automatically a complete check for every frame and shadow-tree arrangement; inspect how the page registers and renders its components.
Choose the right wait instead of guessing
Use whenDefined for late registration
When a script registers the custom element after navigation, whenDefined prevents the capture from racing ahead of registration. It resolves immediately if registration already happened. It does not establish that an instance exists, so query for the element if its presence matters, and fail clearly if it is missing.
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Use application state for completed rendering
A selector or state wait is useful only when it represents the final content contract. Seeing report-card in the DOM, or seeing a “loading” indicator disappear, may still be too early if data, nested components, images, or charts are unfinished. Make the readiness signal correspond to what must actually appear in the PDF.
Do not treat network idle as component readiness
Network idle describes network activity, not whether the application has finished rendering. A page can become quiet while rendering continues, or keep connections open after its printable content is ready. Playwright’s documentation explicitly discourages network-idle as a testing readiness strategy and recommends waiting for the required state instead: Playwright load-state guidance.
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A fixed delay is a timing guess that can be wasteful on a fast run and too short on a slow one. Give the whole operation a finite Go context deadline and wait for an explicit condition. If the condition does not resolve before the deadline, return the error and investigate the page or timeout budget. The chromedp project documents page-state waits and JavaScript evaluation; use the specific wait supported by your pinned version rather than relying on an arbitrary sleep.
Print options and browser requirements
Chromedp drives Chrome or Chromium through the Chrome DevTools Protocol; it is not itself a browser and does not decide when an application component is ready. A compatible browser runtime must be available locally or reachable through the configured browser connection, and the Go context must remain alive through navigation, waiting, and printing.
page.PrintToPDF().Do(ctx) returns PDF bytes and an error. The generated Page API includes print options such as background printing, page dimensions, margins, landscape orientation, and page ranges; check the version you have installed for exact option names and semantics. WithPrintBackground(true) is appropriate when CSS backgrounds are part of the intended output, but print CSS and page layout still determine the result.
Do not transfer behavior from one automation library to another. Puppeteer documents that its Page.pdf() uses print media and waits for fonts by default (Puppeteer PDF API); that does not establish custom-element readiness in chromedp or guarantee that the page’s data and rendering work are complete.
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If you need a clean screenshot or PDF through an API rather than managing a Chrome runtime, ScreenshotNeo provides a one-request website capture API and an MCP server. It does not replace this Go-specific readiness pattern when you need to await an application-owned signal before PDF generation.
Example cURL request using the documented endpoint and parameters:
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The evaluation or PDF call times out
Check whether navigation is waiting on an unexpected page event, whether the page-owned promise is still pending, and whether the Go context deadline is long enough for the target page and browser. Add logging around the application readiness contract or return a descriptive rejection when required data cannot load. Increasing the timeout alone will not fix a promise that never settles.
whenDefined never resolves
Confirm that the exact tag name is valid and that the script responsible for defining it loads successfully in the frame where you are evaluating. If the name is already registered, the promise resolves immediately; a hang usually points to a missing or delayed definition, a stalled page, or a wait running in the wrong frame.
The PDF is missing content despite successful waits
Verify that the readiness promise covers the specific content in question. It may need to await data, nested component rendering, chart completion, required images, or other page work. document.fonts.ready addresses document font loading; it does not wait for those other operations. Also check print-specific CSS and whether backgrounds should be enabled.
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The element exists but its contents are incomplete
Element presence confirms only that a matching node exists. Await the component’s documented ready promise or a page-owned signal that resolves after its printable content has rendered. If the content is in a shadow root, query and assess the relevant state according to that component’s implementation.
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The Go code does not compile against the installed packages
Align chromedp and cdproto versions and inspect the documentation for those exact versions. Generated CDP APIs and options can differ across releases. Also confirm imports, context setup, and the return signature of the PDF command in the version you use.
The browser cannot be started or reached
Install or configure a compatible Chrome/Chromium runtime, or point chromedp at the browser connection your deployment provides. Keep the browser and Go contexts alive until the complete operation returns, and ensure cleanup occurs after capture rather than before the PDF action.
Frequently Asked Questions
Can I use this method when the custom element is already defined?
Yes. The standard promise fulfills immediately when that valid custom element is already defined.
Does awaiting document.fonts.ready mean every image is ready?
No. It addresses document font loading only; include image completion in the page’s readiness contract if images are required for the PDF.
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