How to Debug JSON API Responses: Common Errors & Solutions

Error 1 Unexpected token < in JSON at position 0

Cause: Your client expected JSON, but the endpoint returned an HTML error document (such as a 404 Not Found, 502 Bad Gateway, or Nginx error page beginning with <!DOCTYPE html>).

Solution: Always check response.ok and inspect the Content-Type header before parsing:

const res = await fetch('/api/user');
if (!res.ok) {
  const rawText = await res.text();
  console.error(`API Error (${res.status}):`, rawText);
  throw new Error(`Endpoint returned status ${res.status}`);
}
const data = await res.json();

Error 2 Trailing Commas and Single Quotes

Cause: Unlike JavaScript object literals, strict RFC 8259 JSON forbids trailing commas ({ "a": 1, }) and single-quoted strings ({ 'key': 'value' }).

Solution: Run the payload through our JSON Validator to automatically replace single quotes and eliminate dangling commas.

Error 3 The "Silent Scope Trap": Structural Misalignment

Cause: The JSON is 100% syntactically valid, but an accidentally misplaced closing brace places a critical configuration key under the wrong parent object. (See our Kubernetes Case Study).

Solution: Load the payload into our 2D Node Graph. The visual layout immediately surfaces detached cards that are completely disconnected from their intended parent scopes.