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JS Web APIsLesson #183

JavaScript JSON Values

In JSON, values must be one of the following data types:

JSON Value Types

In JSON, values must be one of the following data types:

Type	Example
String	"John"
Number	30
Object	{"name":"John"}
Array	["red","green"]
Boolean	true
Null	null

JSON Objects

A JSON object is enclosed in curly braces { }.

A JSON object contains one or more properties.

Example

Example Code

javascript

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{

  "name": "John",

  "age": 30,

  "city": "New York"

}

Properties comes in name/value pairs.

Names must be strings enclosed in double quotes.

Values can be any valid JSON value:

Type	Description
String	A sequence of characters enclosed in double quotes
Number	An integer or floating-point number
Boolean	The literal values true or false
Null	The literal value null to indicate the absence of a value
Array	An list of values enclosed in square brackets
Object	A collection of key/value pairs enclosed in curly braces

Property values can be objects.

Example

Example Code

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{
"employee":{"name":"John", "age":30, "city":"New York"}

}

Property values can be arrays:

Example

Example Code

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{
"employees":["John", "Anna", "Peter"]
}

JSON Arrays

A JSON array is enclosed in square brackets [ ].

An array contains an ordered list of values.

Example

Example Code

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[

  "Ford",

  "Volvo",

  "BMW"

]

Array values can be any valid JSON value.

Type	Description
String	A sequence of characters enclosed in double quotes
Number	An integer or floating-point number
Boolean	The literal values true or false
Null	The literal value null to indicate the absence of a value
Array	An list of values enclosed in square brackets
Object	A collection of key/value pairs enclosed in curly braces

Nested Values

Objects and arrays can contain other objects and arrays.

Example

Example Code

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{

  "name": "John",

  "age": 30,

  "address": {

    "city": "New York",

    "country": "USA"

  },

  "hobbies": [

    "Reading",

    "Cycling",

    "Photography"

  ]

}

Nested objects and arrays make it possible to represent complex data structures.

JSON String values

A JSON string value is a sequence of zero or more Unicode characters enclosed in double quotes.

Examples

Example Code

javascript

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""

"Hello World!"

"He said, \"Hello!\""

"\u00A9 2026"

JSON strings represent special characters using a backslash (\) escape syntax:

\" for double quotes
\/ for slash
\\ for backslash
\b for backspace
\f for formfeed
\n for linefeed
\r for carriage return
\t for horizontal tab
\uXXXX for exact Unicode characters

To be valid, JSON strings require double quotes (unlike JavaScript, which permits single quotes).

JSON Number Values

JSON number values can be integers or floating-point numbers.

Example

Example Code

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{

  "age": 30,

  "height": 1.82

  "speed_of_light": 2.997e8

}

JSON does not distinguish between different numeric types. It treats integers and floating-point decimals under a single "number" category.

Type	Description	Example
Integers	Positive or negative whole numbers	-7 42
Fractions	Numbers containing a decimal point followed by at least one digit	-0.5 3.14
Exponents	Numbers using e/E to indicate power of 10	1.0e-2 for 0.01 2.5E3 for 2500

JSON numbers requires absolute structural precision.

Many formats allowed in standard programming languages will break a JSON parser:

Error	Description
No Quotes	"42" turns the value into a JSON string.
No Leading Zeros	Integers can not be padded with zeros (05).
No Leading Plus Signs	Numbers cannot start with plus (+42).
No Number Constants	NaN, Infinity, and -Infinity are invalid.
No Non-Decimal	Hexadecimal and Octal formats are invalid (0x7A).

JSON Boolean Values

JSON supports the Boolean values true and false.

Example

Example Code

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{

  "member": true,

  "student": false

}

Rule	Description
No Quotation	"true" turns the value into a JSON string.
Lowercase	Variations like TRUE, FALSE, True, or False are invalid.
No Numbers	JSON does not accept numbers like 1 or 0 as boolean replacements.

JSON Null

The value null represents an empty value.

Example

Example Code

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{

  "middleName": null

}

undefined Is Not a JSON Value

JSON supports null, but not undefined.

Wrong

Example Code

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{

  "city": undefined

}

Unsupported JavaScript Values

JSON has a small, strict syntax.

These JavaScript values cannot be represented in JSON:

JavaScript Value	JSON
Function	No
Date	No
Symbol	No
BigInt	No
NaN	No
Infinity	No
undefined	No

These values must be converted before they can be stored as JSON.

Date Values

Date objects are not allowed in JSON.

If you need to include a date, write it as a string.

You can convert it back into a date object later:

Example

Example Code

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const text = '{"name":"John", "birth":"1986-12-14"}';

Functions as Values

Wrong

Example Code

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{

  "greet": function() {return "Hello"}

}

If you need to save a function, save it as a string.

You can convert it back into a function later.

Example

Example Code

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const text = '{"name":"John", "age":"function () {return 30;}';

JavaScript syntax and algorithm demonstration.

""

"Hello World!"

"He said, \"Hello!\""

"\u00A9 2026"

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