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Wait for a specific element before capturing in Node.js with built-in fetch

A fixed delay is a guess: too short and you capture the spinner, too long and every render pays for the worst case. wait_for_selector replaces the guess with a condition — pass a CSS selector and the render proceeds the moment a matching element is visible. The dashboard's chart, the map's tiles, the price table: name the thing you are actually waiting for, and wait for that.

Below is a complete, runnable Node.js program using built-in fetch, no third-party dependency needed. It reads your API key from the SNAPDOK_KEY environment variable — free keys take about thirty seconds and need no card.

import { writeFile } from "node:fs/promises";

const res = await fetch("https://snapdok.io/v1/render", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.SNAPDOK_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    url: "https://your-app.com/report/42",
    format: "png",
    wait_for_selector: "#chart-ready",
    wait_until: "networkidle",
  }),
});

if (!res.ok) throw new Error(`${res.status} ${await res.text()}`);
await writeFile("ready.png", Buffer.from(await res.arrayBuffer()));
console.log(res.headers.get("x-cache"), "// the settled render is cached like any other");

Selector choice is the whole game. Pick an element that appears last in your page's load sequence — a #chart-ready marker your own code adds after drawing, the final list item, the element a skeleton placeholder is replaced by. An element that exists in the initial HTML matches immediately and waits for nothing; visibility is required precisely so a pre-rendered-but-hidden node does not count as ready.

The failure mode is the feature: if the element never becomes visible within timeout, the render fails with 504 SELECTOR_TIMEOUT — not metered, and unambiguous in your logs — instead of silently shipping a half-rendered image the way an expired delay does. The sequencing composes with the other waits: navigation finishes (wait_until), then delay runs, then the selector wait begins. It works on every format, pdf_forms included.

Same endpoint, one more trick: if the page you are rendering has a form on it, adding "pdf_forms": true to a PDF render brings it back with real, fillable AcroForm fields — a PDF people can type into, not a picture of one. How that works.

Notes for built-in fetch

No dependency needed: fetch is global from Node 18 onward (snapdok's own test suite uses it). The binary-safety detail is res.arrayBuffer() → Buffer.from(...) — reaching for res.text() is the classic way to corrupt a PDF in Node. fetch does not reject on HTTP error statuses, so the res.ok check is load-bearing: without it, an expired key writes a 40-byte JSON error to disk with a .pdf extension.

Response headers worth reading

Header	Meaning
`X-Cache`	HIT when an identical request rendered within 24h.
`X-RateLimit-Remaining`	Requests left in the current second.

Full parameter reference: the docs. Hard numbers on caps and timeouts: limits.

Wait for a specific element before capturing in Node.js with built-in fetch

Notes for built-in fetch

Response headers worth reading

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