Common JSON Errors and How to Fix Them

JSON's grammar is deliberately tiny - and strictly enforced. Most parse errors happen because JSON looks like JavaScript but forbids half of JavaScript's conveniences. Below are the seven classics, each with a broken and fixed version. When you just want the answer, paste your text into our JSON Formatter - it validates, points at the failing position, and pretty-prints or minifies, entirely in your browser.

1. Trailing Commas

JavaScript tolerates a comma after the last item; JSON never does. This is the single most common error when hand-editing config files.

// Before - invalid
{ "name": "Ada", "age": 36, }

// After - valid
{ "name": "Ada", "age": 36 }

2. Single Quotes

JSON strings use double quotes only - for keys and values alike. Apostrophes inside a double-quoted string are fine.

// Before - invalid
{ 'title': 'It\'s ready' }

// After - valid
{ "title": "It's ready" }

3. Unquoted Keys

Object keys must be quoted strings, even simple identifier-like ones.

// Before - invalid
{ status: "active", count: 3 }

// After - valid
{ "status": "active", "count": 3 }

4. Missing Brackets or Braces

Every { needs its } and every [ its ] - deep nesting makes the missing one hard to spot by eye, and the error often points at the end of the file.

// Before - invalid (array never closed)
{ "items": [1, 2, 3, "next": true }

// After - valid
{ "items": [1, 2, 3], "next": true }

A formatter is the fast fix here: pretty-printing re-indents by nesting depth, so the structure that never closed becomes visually obvious.

5. Comments

Standard JSON has no comments, by design. Some editors accept JSONC (JSON with comments) for their own config files, which trains bad habits for everywhere else.

// Before - invalid
{
  // retry twice
  "retries": 2
}

// After - valid
{
  "_comment": "retry twice",
  "retries": 2
}

6. NaN, undefined, and Friends

Valid JSON values are exactly: string, number, object, array, true, false, null. JavaScript's undefined, NaN, and Infinity are not on the list.

// Before - invalid
{ "score": NaN, "rank": undefined }

// After - valid
{ "score": null, "rank": null }

7. BOM and Invisible Characters

The sneakiest one: the file looks perfect but fails on the first character. Files saved as "UTF-8 with BOM" begin with three invisible bytes (EF BB BF) that strict parsers reject, and text pasted from Word or chat apps carries curly quotes (“ ”) and non-breaking spaces that are not the ASCII characters JSON requires. Fix: save as UTF-8 without BOM, and re-type any quotes that came from a document. A validator that reports "unexpected character at position 0" is almost always describing a BOM.

JSON Is Not a JavaScript Object Literal

The root cause behind errors 1-6 is one misconception. JSON was derived from JavaScript syntax, but it is a stricter, language-independent data format: double quotes only, quoted keys, no trailing commas, no comments, no expressions, and only the seven value types. Every object literal your JavaScript accepts is not necessarily JSON - though every JSON document is (almost) valid JavaScript.

The practical rule: never build JSON by concatenating strings by hand. Construct a real object and let JSON.stringify() serialize it - the output is guaranteed valid - and parse with JSON.parse() rather than eval(), which would happily execute malicious input. Hand-editing is fine for config files; just run the result through a validator before deploying, because a single trailing comma can take down a build.

Frequently Asked Questions