Common JSON Errors and How to Fix Them

Every JSON developer hits these at some point. Here’s a reference for the errors that come up most often, with invalid and corrected examples.

Tip: paste your JSON into the JSON Validator to get the exact line and column of the error.

1. Missing Comma Between Elements

Every key-value pair in an object (and every item in an array) must be separated by a comma, except the last one.

❌ Invalid

{
  "name": "Alice"
  "age": 30
}

✅ Valid

{
  "name": "Alice",
  "age": 30
}

JSON does not allow a comma after the last item in an object or array. This differs from JavaScript and JSON5.

❌ Invalid

{
  "name": "Alice",
  "age": 30,
}

✅ Valid

{
  "name": "Alice",
  "age": 30
}

JSON requires double quotes for both keys and string values. Single quotes are valid in JavaScript but not in JSON.

❌ Invalid

{'name': 'Alice'}

✅ Valid

{"name": "Alice"}

Unlike JavaScript object literals, JSON requires all keys to be wrapped in double quotes.

❌ Invalid

{name: "Alice"}

✅ Valid

{"name": "Alice"}

Standard JSON does not support comments (// or /* */). Use JSON5 or strip comments before parsing.

❌ Invalid

{
  // This is a comment
  "name": "Alice"
}

✅ Valid

{
  "name": "Alice"
}

undefined, NaN, and Infinity are JavaScript values but not valid JSON. Use null, or convert numbers before serializing.

❌ Invalid

{"value": undefined, "ratio": NaN}

✅ Valid

{"value": null, "ratio": null}

Every opening brace { must have a matching }, and every opening bracket [ must have a matching ]. Mixed delimiters always fail.

❌ Invalid

{"items": [1, 2, 3}

✅ Valid

{"items": [1, 2, 3]}

Backslashes in strings must be escaped as \\. Similarly, double quotes inside strings must be escaped as \".

❌ Invalid

{"path": "C:\Users\name"}

✅ Valid

{"path": "C:\\Users\\name"}