JSON Debugging Sheet

The five things that break JSON

Trailing comma

{"a":1,} — reads as 'expected property name'

Single quotes

{'a':1} — strings need double quotes

Unquoted key

{a:1} — keys must be quoted strings

Comments

// and /* */ are not JSON

Raw line break

inside a string — escape it as \n

What JSON actually allows

Types

object, array, string, number, true, false, null

No

undefined, NaN, Infinity, dates, functions

Numbers

no leading +, no leading zeros, no hex

Strings

double quotes only, backslash escapes

Top level

any value — not just an object or array

Key order

preserved in practice, not guaranteed by spec

Finding the break

• Read the position in the error - then look at the character BEFORE it
• The message says what the parser EXPECTED, not what you got wrong
• 'Expected property name' after a comma = trailing comma
• Check the LAST item of every object and array
• Search for single quotes
• Search for // and /*
• Check anything pasted from a spreadsheet for smart quotes
• Check strings that span lines - the break must be escaped
• If it came from JavaScript, remember JS is far more forgiving than JSON

The error IS the output

When the document does not parse, both boxes show the parser's own message rather than failing silently. That message names what it expected and where it stopped - which, once you know it points just past the mistake rather than at it, is usually enough to find the character responsible in a few seconds.

Careful what you paste anywhere

This runs entirely in your browser and uploads nothing. Not every online JSON tool does, and the things people paste into them are frequently production API responses and config files with credentials still in them. Worth a habit of checking before pasting, wherever you are.