Common JSON Syntax Errors and How to Fix Them

JSON is stricter than JavaScript object literals

A common source of JSON errors is treating JSON like JavaScript. JSON has a smaller grammar: object keys must be quoted, strings must use double quotes, comments are not allowed, and trailing commas are invalid.

Frequent mistakes

• Missing comma: every property or array item must be separated from the next one.
• Unquoted key: use "name" instead of name.
• Single quotes: use "value" instead of 'value'.
• Trailing comma: remove the comma after the last object property or array item.
• Comment lines: remove // and /* */ comments before parsing as JSON.
• Broken nesting: check that every { has a matching } and every [ has a matching ].

A practical debugging workflow

Start with the first parser error instead of fixing the whole file at once. A missing comma near the top can create misleading errors later in the document.

After the JSON validates, run it through a formatter. Indentation makes mismatched nesting and unexpected object shapes much easier to see.

Malformed and corrected payload

Invalid:  { name: 'Ada', active: true, }
Correct:  {"name":"Ada","active":true}

Invalid:  {"items":[1 2]}
Correct:  {"items":[1,2]}

Debugging sequence

• Start at the first parser position, not the last visible error.
• Check the previous string delimiter, comma, or bracket.
• Reduce nested payloads while preserving the failing line.
• Validate the corrected payload again before testing its schema.

Why the reported position can look wrong

Parsers usually report where they can no longer continue, not necessarily where the mistake began. A missing closing quote on one line can make the next property name appear to be invalid. A missing comma can be reported at the first character of the following value. Read backward from the reported position and verify the nearest quote, comma, colon, bracket, and brace.

Reducing the payload is often faster than visually scanning a deeply nested document. Preserve the failing region and remove unrelated siblings in small steps. If the error disappears, restore the last removed section. This produces a minimal malformed example that can be attached to a bug without exposing the original payload.

Strict JSON versus convenient editor formats

JSON does not allow comments, trailing commas, unquoted property names, single-quoted strings, undefined, NaN, or hexadecimal numeric literals. Some editors accept JSON with Comments or a JavaScript object literal and still label the file JSON. An API parser is entitled to reject those extensions.

Encoding errors can masquerade as syntax problems. A byte-order mark, invalid UTF-8, or an unexpected control character may be invisible in a normal editor. When a payload looks correct, inspect the exact bytes and Content-Type charset before rewriting valid-looking fields.

Error-focused QA cases

• Missing comma between adjacent properties and array items.
• Trailing comma after the last property or item.
• Unescaped quote, newline, tab, or backslash inside a string.
• Mismatched brace/bracket at two different nesting depths.
• Duplicate keys, tested according to the receiving application's documented policy.
• A valid JSON body sent with the wrong Content-Type or character encoding.

References

• RFC 8259: JSON Grammar and Strings
• MDN JSON.parse error reference

FAQ

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