Your JSON Is Valid... Until It's Not: Common Parse Errors and How to Fix Them

We've all seen it: an API response that looks perfectly fine, but JSON.parse() throws a cryptic error at position 0. Or position 472. Or somewhere deep inside a 10,000-line config file. Here are the most common JSON syntax issues and how to diagnose them fast.

1. Trailing Commas (The #1 Offender)

{
  "name": "App",
  "version": "2.0.1",
  "dependencies": [
    "react",
    "lodash",
  ],
}

JSON spec (RFC 8259) does not allow trailing commas. JavaScript and TypeScript are forgiving, so your IDE doesn't complain — but strict JSON parsers will fail. The fix: remove the comma after the last array item and last object key.

2. Smart Quotes vs. Straight Quotes

If you copy JSON from a word processor or a ChatGPT output, you might get "smart quotes" (curly) instead of straight quotes. They look similar but are different Unicode characters:

// This WILL fail — smart quotes on the key
{“name”: “hello”}

// This is correct
{"name": "hello"}

3. Unquoted Keys

JavaScript objects let you use unquoted keys. JSON does not:

// Valid JS object, INVALID JSON
{name: "Alice", age: 30}

// Correct JSON
{"name": "Alice", "age": 30}

4. The Invisible BOM

If your JSON file starts with a UTF-8 BOM (Byte Order Mark), many parsers fail at position 0 — even though the file looks empty of problems. cat file.json | xxd | head -1 and look for EF BB BF at the start.

5. Single Quotes

JavaScript accepts both single and double quotes for strings. JSON does not:

// Invalid
{'key': 'value'}

// Valid
{"key": "value"}

Debugging Tip

When you encounter a parse error, don't just stare at the JSON. Use a formatter that shows the exact line and column of the first syntax error.

I built a free JSON formatter and validator for exactly this purpose — paste your JSON, click Validate, and it tells you exactly where the error is. All processing happens in your browser, no data leaves your machine.

The most useful feature: it works offline once the page loads. I've used it on flights to debug local config files without internet.

Got a favorite JSON debugging technique? Drop it in the comments.