Reading and Debugging JSON: A Practical Guide
2026-06-03 · 5 min read
Trailing commas, single quotes, missing brackets — the usual suspects behind "Unexpected token". Here is how to fix them fast.
JSON (JavaScript Object Notation) is the lingua franca of APIs. Its rules are strict, which is good for machines but means a single stray character breaks the whole document. Knowing the common pitfalls makes debugging quick.
The core syntax rules
- Keys and string values must use double quotes — never single quotes.
- No trailing commas after the last item in an object or array.
- Values are strings, numbers, booleans, null, objects, or arrays — nothing else.
- No comments are allowed in standard JSON.
The errors you will actually hit
The infamous "Unexpected token" almost always means a trailing comma, a single quote, an unquoted key, or a missing bracket. The error usually reports a position — start reading just before it.
Prettify to see structure
Minified JSON on one line is unreadable. Formatting it with indentation reveals nesting and makes a missing or extra bracket obvious. Sorting keys alphabetically helps when comparing two payloads.
When an API response will not parse, paste it into a validator first. It is faster than scanning by eye and points you straight at the offending character.
Format and validate
The JSON Formatter prettifies, minifies, and validates JSON with inline error highlighting and a character position, so you can fix broken payloads in seconds.