JSON Validator
Validate JSON syntax and identify errors in your JSON data. This tool checks for proper formatting, missing brackets, invalid characters, and other common JSON mistakes.
Common JSON Errors
| Error | Cause | Fix |
|---|---|---|
| Unexpected token | Syntax error | Check for typos |
| Unterminated string | Missing quote | Close all strings |
| Trailing comma | Comma after last item | Remove trailing comma |
| Invalid character | Non-JSON character | Use proper escaping |
| Missing colon | Key without value | Add : after key |
Valid JSON Rules
``javascript
// Keys must be strings with double quotes
{ "name": "John" } // ✓ Valid
{ name: "John" } // ✗ Invalid
// Strings must use double quotes
{ "name": "John" } // ✓ Valid
{ "name": 'John' } // ✗ Invalid
// No trailing commas
{ "a": 1, "b": 2 } // ✓ Valid
{ "a": 1, "b": 2, } // ✗ Invalid
// No comments
{ "a": 1 } // ✓ Valid
{ "a": 1 } // note // ✗ Invalid
`
Validation Code
`javascript
function validateJSON(jsonString) {
try {
JSON.parse(jsonString);
return { valid: true };
} catch (error) {
return {
valid: false,
error: error.message,
position: error.message.match(/position (\d+)/)?.[1]
};
}
}
// Example
const result = validateJSON('{"name": "John", }');
// { valid: false, error: "Unexpected token }", position: 17 }
`
JSON Data Types
| Type | Example | Notes |
|---|---|---|
| String | "hello" | Double quotes only |
| Number | 42, 3.14 | No quotes, no leading zeros |
| Boolean | true, false | Lowercase only |
| Null | null | Lowercase only |
| Array | [1, 2, 3] | Square brackets |
| Object | {"a": 1} | Curly braces |
Use this validator to ensure your JSON is syntactically correct.Numbers Are the Recurring Problem
A JSON number is an IEEE 754 double. Three consequences that bite in production:
| Value | What happens |
|---|---|
| Integers above 2⁵³ | Silently lose precision — send IDs as strings |
| Money as a float | 0.1 + 0.2 = 0.30000000000000004 |
| Leading zeros | 007 is invalid JSON; "007" is a string |
NaN and Infinity | Not valid JSON at all |
Twitter hit the first one publicly: 64-bit tweet IDs arrived in JavaScript rounded, so the
API began sending an id_str alongside every id.Keys, Order and Duplicates
Objects are formally unordered, though every JavaScript engine preserves insertion order for
string keys — with one exception: integer-like keys sort numerically and come first.
`javascript
JSON.stringify({ b: 1, 2: 2, a: 3 }); // {"2":2,"b":1,"a":3}
`
Duplicate keys are not an error in the spec, and JSON.parse keeps the last one. Two
parsers can legitimately disagree about which value wins, which has been the basis of real
request-smuggling attacks.
Rules That Prevent Most JSON Bugs
| Rule | Why |
|---|---|
| No trailing commas | {"a": 1,} is invalid JSON, though JavaScript accepts it in object literals |
| Double quotes only | {'a': 1} is not JSON |
| Keys must be quoted | {a: 1} is a JavaScript object, not JSON |
| No comments | There is no comment syntax; JSON5 and JSONC are different formats |
No undefined, NaN, Infinity | JSON.stringify silently drops or nulls them |
| Numbers are IEEE 754 doubles | Integers above 2⁵³ lose precision — send them as strings |
Parsing Safely
`javascript
try {
const data = JSON.parse(text);
} catch (error) {
// The message names the character offset, which is the fastest way
// to find an unescaped quote in a large document.
console.error(error.message);
}
`
JSON.parse` throws on invalid input rather than returning null, so it always belongs in a try/catch when the source is a file, a request body or a clipboard paste.