JSON errors are among the most frequent issues developers encounter when working with APIs, configuration files, and data exchange. While the JSON specification is simple, small syntax mistakes can waste hours of debugging time.

Here are the 10 most common JSON errors — and how to fix each one.

Error: A comma after the last element in an object or array.

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

Fix: Remove the comma after the last item.

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

JavaScript allows trailing commas in object literals, but JSON strictly does not.

Error: Using single quotes for keys or string values.

{ 'name': 'Alice' }

Fix: Replace all single quotes with double quotes.

{ "name": "Alice" }

This is the most common mistake when copying JavaScript objects into a JSON context.

Error: Object keys without surrounding double quotes.

{ name: "Alice" }

Fix: Always wrap keys in double quotes.

{ "name": "Alice" }

Error: Missing commas between properties or array elements.

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

Fix: Add a comma after each property (except the last).

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

Error: A comma in the wrong place, like before a closing bracket.

[1, 2, 3,]

Fix: Remove the comma before the closing bracket.

[1, 2, 3]

Error: Unmatched opening braces or brackets without corresponding closing ones.

{ "items": [1, 2, 3

Fix: Ensure every opening { has a closing }, and every [ has a ].

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

Error: JSON does not support comments — not even // or /* */.

{ "name": "Alice" /* user's name */ }

Fix: Remove all comments from your JSON data.

{ "name": "Alice" }

If you need to add notes, consider using a _comment key in your object instead.

Error: Using Python's boolean or null values instead of JavaScript JSON equivalents.

{ "active": True, "data": None }

Fix: Use lowercase true, false, and null.

{ "active": true, "data": null }

Error: Using JavaScript's undefined in JSON data.

{ "name": undefined }

Fix: Use null instead of undefined.

{ "name": null }

Unlike JavaScript, JSON does not have an undefined type.

Error: Numbers with leading zeros, hex values, or special number formats.

{ "value": 0123, "hex": 0xFF }

Fix: Remove leading zeros and use only decimal numbers.

{ "value": 123 }