JSON Syntax Guide: Rules, Examples, and Common Errors

JSON is one of the most common formats used to share data between applications. You will find it in API responses, configuration files, webhooks, JavaScript applications, and many developer tools.

It looks simple, but one missing quote, comma, or bracket can make the entire JSON file invalid.

This guide explains JSON syntax in plain language. You will learn the main rules, understand JSON data types, see valid examples, and know how to fix common JSON errors.

JSON stands for JavaScript Object Notation. It is a lightweight text format used to store and exchange structured data.

For example, an API may return user information in JSON like this:

{
  "id": 101,
  "name": "Example User",
  "email": "user@example.com",
  "active": true
}

JSON is easy for people to read and easy for software to process. Although its name includes JavaScript, JSON is used with many programming languages, including PHP, Python, Java, Laravel, React, Node.js, and Java.

Valid JSON follows a few strict rules:

1. Data is written as key-value pairs.

2. Keys must be enclosed in double quotes.

3. String values must use double quotes.

4. Each key-value pair is separated by a comma.

5. Objects use curly brackets: { }.

6. Arrays use square brackets: [ ].

7. JSON does not allow trailing commas.

8. Comments are not allowed in standard JSON.

Here is a simple valid JSON object:

{
  "product": "Sample Product",
  "price": 2499,
  "in_stock": true
}

In this example:

• "product" is a key.

• "Sample Product" is a string value.

• 2499 is a number.

• true is a boolean value.

A JSON object contains related data inside curly brackets.

{
  "name": "Example User",
  "role": "Developer",
  "city": "Sample City"
}

Every item in an object has a key and a value.

"key": value

Keys should be unique within the same object. Avoid using the same key twice because applications may handle duplicate keys differently.

An array is a list of values. Arrays use square brackets.

{
  "languages": [
    "PHP",
    "JavaScript",
    "Python"
  ]
}

Arrays can contain strings, numbers, objects, booleans, or even other arrays.

Here is an array of objects:

{
  "users": [
    {
      "id": 1,
      "name": "Example User One"
    },
    {
      "id": 2,
      "name": "Example User Two"
    }
  ]
}

This format is common in API responses when an application needs to return multiple records.

JSON supports six main value types.

String

A string is text inside double quotes.

{
  "message": "Welcome to DevToolbox"
}

Correct:

{
  "city": "Sample City"
}

Incorrect:

{
  "city": Sample City
}

The value must have double quotes because it is text.

Number

Numbers do not use quotes.

{
  "quantity": 5,
  "price": 799.99
}

If you write a number inside quotes, it becomes a string instead.

{
  "quantity": "5"
}

This is valid JSON, but it may cause problems if your application expects a number.

Boolean

A boolean value is either true or false.

{
  "email_verified": true,
  "newsletter_subscribed": false
}

Boolean values must be lowercase. True and False are not valid JSON.

Null

Use null when a value is intentionally empty or unavailable.

{
  "middle_name": null
}

Do not wrap null in quotes unless you actually want the text "null".

Object

An object contains related key-value data.

{
  "address": {
    "city": "Sample City",
    "state": "Sample State",
    "country": "Example Country"
  }
}

Array

An array stores a list of values.

{
  "skills": [
    "WordPress",
    "Laravel",
    "React"
  ]
}

Here is a realistic example of JSON returned by an ecommerce API:

{
  "order_id": 5001,
  "customer": {
    "name": "Example User",
    "email": "user@example.com"
  },
  "items": [
    {
      "name": "Sample Product",
      "quantity": 1,
      "price": 1499
    },
    {
      "name": "Sample Accessory",
      "quantity": 2,
      "price": 299
    }
  ],
  "total": 2097,
  "payment_status": "paid",
  "delivered": false
}

This example contains objects, arrays, strings, numbers, and booleans together.

JSON is strict. These are some of the most common mistakes developers make.

1. Using Single Quotes Instead of Double Quotes

This is one of the most common errors, especially for developers who work with JavaScript or PHP arrays.

Invalid JSON:

{
  'name': 'Example User'
}

Valid JSON:

{
  "name": "Example User"
}

JSON requires double quotes around keys and string values.

2. Adding a Trailing Comma

A trailing comma is a comma after the final item in an object or array.

Invalid JSON:

{
  "name": "Example User",
  "city": "Sample City",
}

Valid JSON:

{
  "name": "Example User",
  "city": "Sample City"
}

3. Forgetting a Comma Between Items

Each item in an object or array must be separated by a comma.

Invalid JSON:

{
  "name": "Example User"
  "city": "Sample City"
}

Valid JSON:

{
  "name": "Example User",
  "city": "Sample City"
}

4. Adding Comments

Standard JSON does not support comments.

Invalid JSON:

{
  "name": "Example User",
  // This is the customer name
  "city": "Sample City"
}

If you need comments in a configuration file, consider using another format that supports them. Do not add comments to JSON sent through an API.

5. Not Closing Brackets or Quotes

Every opening {, [, or " must have a matching closing character.

Invalid JSON:

{
  "skills": [
    "PHP",
    "Laravel"
  }

Valid JSON:

{
  "skills": [
    "PHP",
    "Laravel"
  ]
}

When JSON is long or nested, it can be difficult to spot a syntax error manually.

Paste your JSON into the DevToolbox JSON Formatter and Validator. It can format readable JSON and highlight syntax issues, making it easier to find missing commas, quotes, brackets, and invalid values.

This is especially useful when you are:

• Testing an API response

• Creating a webhook payload

• Editing a configuration file

• Working with a large JSON export

• Debugging a frontend or backend request

JSON and JavaScript objects look similar, but they are not exactly the same.

JavaScript objects can use single quotes, unquoted keys, functions, comments, and trailing commas in some situations. JSON cannot.

For example, this may work in JavaScript:

const user = {
  name: 'Example User',
  active: true,
};

But it is not valid JSON.

The JSON version must be:

{
  "name": "Example User",
  "active": true
}

When sending data through APIs, always use valid JSON.

Before using JSON in an API or configuration file, check the following:

• Are all keys inside double quotes?

• Are all text values inside double quotes?

• Is every item separated by a comma?

• Is there no comma after the final item?

• Are all brackets and braces closed correctly?

• Are true, false, and null lowercase?

• Have all comments been removed?

• Does the JSON pass validation?