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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →If a QR API accepts a JSON request but returns a PNG, response.json() will fail: the response body contains image bytes, not JSON. Check the HTTP status, read a successful PNG response with arrayBuffer(), and convert it to a Node.js Buffer. Request and response formats are separate parts of an API contract.
Why does response.json() fail when a QR API returns a PNG?
The request header Content-Type: application/json describes the body your code sends. It does not determine the format the server sends back. A QR endpoint can accept JSON describing the text and desired format, then return binary PNG bytes on success and JSON only when reporting an error. LinkScan documents that kind of contract: JSON input, PNG or SVG success output, and JSON error responses for 4xx and 5xx results. LinkScan API documentation.
Calling response.json() on PNG bytes asks JavaScript to parse image data as JSON, which produces a parsing error. Read the successful response according to the selected output format instead. QRGen documents PNG, SVG, and a JSON mode containing a modules matrix and size value; that JSON is structured QR data, not a PNG file. QRGen Node.js guide.
How to read a PNG response in Node.js
For Node.js 18 and later, native fetch is available. The following pattern checks for failure before reading the successful PNG body as bytes:
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import { writeFile } from "node:fs/promises";
const response = await fetch(apiUrl, {
method: "POST",
headers: { "Content-Type": "application/json" },
body: JSON.stringify({ data: "https://example.com", format: "png" }),
});
if (!response.ok) {
// Use this only when the API documents JSON error responses.
const error = await response.json();
throw new Error(error.error ?? `QR API failed: ${response.status}`);
}
const png = Buffer.from(await response.arrayBuffer());
await writeFile("qr.png", png);The JSON error parsing in this example is appropriate only if the provider documents JSON errors. APIs differ; if the endpoint uses another error format, handle that contract instead. Checking response.ok first prevents a documented JSON error from being saved or forwarded as though it were an image. The PNG success path follows the byte-reading pattern in the QRGen Node.js guide.
How to relay the PNG through an Express route
Keep the image as a buffer when sending it to a browser or another client. Set the outgoing media type to image/png so the receiver interprets the body as an image; do not JSON-serialize the buffer.
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app.get("/qr.png", async (req, res, next) => {
try {
const response = await fetch(apiUrl, requestOptions);
if (!response.ok) {
return res.status(response.status).send("QR generation failed");
}
const png = Buffer.from(await response.arrayBuffer());
res.type("image/png").send(png);
} catch (error) {
next(error);
}
});Choose caching behavior for the specific QR payload and endpoint rather than copying a universal cache duration: whether an image is safe and useful to cache depends on whether its contents are public and stable. QRGen documents Express forwarding with an image media type in its Node.js guide.
Choose a response reader for the output format
| API output | Read it with | What you receive | Typical use |
|---|---|---|---|
| PNG | arrayBuffer(), then Buffer.from() |
Binary raster image | Fixed-size image output or a system that expects PNG |
| SVG | text() |
Text markup | Scalable output, including print workflows where the receiving application accepts SVG |
| JSON | json() |
Structured data, if the provider explicitly offers a JSON response mode | Applications that need the QR matrix or other documented structured fields rather than an image file |
QRGen documents these PNG, SVG, and JSON options, while UseQR describes SVG as suitable for resizing and print use. Confirm the provider’s output contract and the receiving application’s requirements before choosing a format. QRGen Node.js guide; UseQR Node.js guide.
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Hosted QR API or local Node.js generation?
A hosted API can provide generation without adding and maintaining a rendering dependency in your application, and may offer features your local implementation would otherwise need to build. Local generation with the qrcode package avoids sending the payload to a remote API and can work without network access. UseQR documents local PNG file output, data URLs, SVG strings, and streaming PNG to an HTTP response. These are implementation tradeoffs, not evidence of a performance advantage for either option. UseQR Node.js guide.
Consider where QR content goes before choosing a hosted service. A code may encode Wi-Fi credentials, personal contact information, payment details, or other sensitive data. UseQR recommends local generation for sensitive payloads; that is vendor guidance, not an independent security audit. For a hosted API, review its data handling and retention terms rather than assuming the payload is private.
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Common QR API response errors and fixes
- JSON parse error on a successful response: The endpoint may have returned PNG bytes. Use
arrayBuffer()and convert the result to aBuffer. - Broken image after forwarding: Keep the response body binary and send it with the outgoing
Content-Type: image/png. - SVG treated as an image buffer: Read SVG output with
text()and return it with the SVG media type. - Error body handled as an image: Check
response.okbefore taking the success path, and parse failures according to the provider’s documented error format. - Assuming another provider behaves the same: Check its current endpoint, authentication, supported formats, limits, and error contract. For example, LinkScan documents bearer authentication and a limit of 60 requests per minute for its API; QRGen’s guide describes its example API as requiring no API key. Those details are provider-specific, not general QR API rules. LinkScan API documentation; QRGen Node.js guide.
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