JavaScript JSON Syntax
JSON has a very simple syntax for representing structured data.
One JSON document consists of one JSON value.
The value can be:
- An Object
- An Array
- A String
- A Number
- A Boolean
- or null
JSON Object Literals
This is a JSON string:
'{"name":"John", "age":30, "car":null}'
Inside the JSON string there is a JSON object literal:
{"name":"John", "age":30, "car":null}
A JSON object is written between curly braces { }.
An object contains zero or more properties.
Properties comes in key/value pairs separated by commas.
key names are strings enclosed in double quotes.
Keys and values are separated by a colon.
Values must be a valid JSON data type:
- string
- number
- object
- array
- boolean
- null
It is a common mistake to call a JSON object literal "a JSON object".
JSON cannot be an object. JSON is a string format.
The data is only JSON when it is in a string format.
When a JSON object litteral is converted into a JavaScript variable, it becomes a JavaScript object.
JSON Array Literals
This is a JSON string:
'["Ford", "BMW", "Fiat"]'
Inside the JSON string there is a JSON array literal:
["Ford", "BMW", "Fiat"]
Other Examples
[
"Apple",
"Banana",
"Orange"
]
[1, 2, 3, 4 ,5]
A JSON array is written between square brackets [ ].
An array contains zero or more values.
Values are separated by commas.
Values must be a valid JSON data type:
- string
- number
- object
- array
- boolean
- null
JSON Values
JSON supports six value types.
| Value | Example |
|---|---|
| String | "John" |
| Number | 42 |
| Boolean | true |
| Null | null |
| Object | {"name" : "John"} |
| Array | [1, 2, 3] |
JSON syntax is derived from JavaScript object notation syntax:
- Curly braces hold objects
- Properties comes is in name/value pairs
- Properties are separated by commas
- Square brackets hold arrays
- Strings are in double quotes
But, JSON has stricter syntax rules than JavaScript objects.
JSON Does Not Allow
JSON has a small, strict syntax.
The following JavaScript features are not valid JSON:
| Feature | JSON | JS |
|---|---|---|
| Unquoted property names | No | Yes |
| Single quoted strings | No | Yes |
| Trailing commas | No | Yes |
| Comments | No | Yes |
JSON Property Names
Every property name must be a string.
Property names must be enclosed in double quotes.
Valid JSON
{
"name": "John"
}
Invalid JSON
{
name: "John"
}
JSON Strings
JSON strings must use double quotes.
Valid JSON
{
"city": "London"
}
Invalid JSON
{
"city": 'London'
}
Whitespace
Whitespace is ignored outside strings.
You can use spaces, tabs, and line breaks to make JSON easier to read.
Example
{"name":"John","age":30}
Equivalent JSON
{
"name": "John",
"age": 30
}
Trailing Commas
JSON does not allow a comma after the last property or array value.
Wrong
{
"name": "John",
"age": 30,
}
Correct
{
"name": "John",
"age": 30
}
Comments Are Not Allowed
JSON does not support comments.
Wrong
{
// Customer name
"name": "John"
}
