TL;DR
- → JSON keys must be double-quoted strings. Trailing commas and comments are not valid.
- → The six valid value types: string, number, boolean, null, array, object.
- → Validate and format your JSON instantly with the free JSON Formatter.
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JSON powers virtually every modern API, config file, and data pipeline. You'll encounter it in REST APIs, package.json, environment configs, database exports, and localStorage. Learning to read, write, and debug it confidently is a foundational skill — and it takes about 20 minutes.
What Valid JSON Looks Like
Here is a complete, valid JSON object representing a user:
{
"id": 1,
"name": "Justin Pirrie",
"email": "justin@toolstack.tech",
"isPro": false,
"tags": ["seo", "developer", "marketer"],
"address": {
"city": "London",
"country": "UK"
},
"deletedAt": null
}Every element here demonstrates one of JSON's six valid value types.
The 6 Valid Value Types
| Type | Example | Notes |
|---|---|---|
| String | "Hello, world" | Must use double quotes — single quotes are invalid |
| Number | 42 or 3.14 | No quotes. Supports integers and floats |
| Boolean | true or false | Lowercase only — True is invalid |
| Null | null | Lowercase only — NULL or Null are invalid |
| Array | ["a", "b", "c"] | Ordered list, any mix of value types |
| Object | { "key": "value" } | Unordered set of key/value pairs |
The 5 Most Common JSON Errors
Trailing comma
The comma after the last property (or last array item) is invalid in JSON. JavaScript allows it; JSON does not.
Single-quoted strings
JSON requires double quotes for both keys and string values. Single quotes are a JavaScript convention, not a JSON standard.
Unquoted keys
Every key in a JSON object must be a double-quoted string — no exceptions. This catches people coming from JavaScript object literals.
Comments
JSON has no comment syntax. Remove all // and /* */ comments before parsing. Use JSONC or a pre-processor if you need annotated config files.
undefined value
undefined is a JavaScript concept and is not a valid JSON value type. Use null to represent the absence of a value.
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Validate your JSON in one click
The free JSON Formatter catches syntax errors with exact line positions, formats your payload, and runs entirely in your browser. No data sent anywhere.
Open JSON Formatter Free →Frequently Asked Questions
What is JSON used for?
JSON (JavaScript Object Notation) is used to transmit structured data between a server and a client — it is the standard format for REST API responses. It is also widely used for configuration files (package.json, tsconfig.json), storing user preferences, and exporting/importing data between systems.
What is the difference between JSON and JavaScript objects?
JSON looks like a JavaScript object but has stricter rules: all keys must be double-quoted strings, trailing commas are not allowed, and values can only be strings, numbers, booleans, null, arrays, or nested objects. JavaScript objects allow unquoted keys, single quotes, and trailing commas. JSON is a data format; a JavaScript object is a runtime data structure.
Why does JSON not support comments?
JSON was designed by Douglas Crockford specifically as a minimal data interchange format. Comments were excluded intentionally to keep parsing simple and deterministic. If you need comments in config files, consider JSONC (JSON with Comments) which is supported by TypeScript's tsconfig.json, or YAML.
How do I validate JSON online?
The easiest way is to paste your JSON into the ToolStack JSON Formatter. It will instantly highlight syntax errors with exact line positions, show a formatted version, and allow you to minify or copy the clean output. It runs entirely in your browser — no data is sent to a server.
What is the difference between JSON.parse() and JSON.stringify()?
JSON.parse() converts a JSON string into a JavaScript object — you use it when you receive data from an API. JSON.stringify() converts a JavaScript object into a JSON string — you use it when you need to send data or store it. A common pattern: fetch('/api/data').then(r => r.json()) uses JSON.parse() internally.