15 15. DATA TYPES IN JSON
Individual pieces of data in a JSON file must be one of the following simple data types: string, number, boolean and null (these are described in the following section). In addition, multiple data values can be arranged into more complex structures, i.e. objects and arrays (see below).
15.1 “PRIMATIVE” (or SIMPLE) JSON DATATYPES (string, number, boolean, null)
Each individual piece of data in a JSON file must be one of the following data types: string, number, boolean, null. Technically a file is a valid JSON file if it contains just a single number, a single string, a single true or false or a single null. Practically speaking though, most JSON files arrange multiple pieces of data into objects and arrays as described in the next section.
The simple (or primative) datatypes are:
- "string": string is just another way of saying "character" data.
Strings in JSON must be enclosed in "double quotes" (i.e. quotation marks).
You may NOT use 'single quotes' (i.e. apostrophes) to enclose strings.
Strings may contain the following "backslash escape sequences", i.e.
characters preceded by a backslash:
\" - Double quote \\ - Backslash
\/ - Forward slash \b - Backspace
\f - Form feed \n - Newline
\r - Carriage return \t - Tab
\u - followed by four hexadecimal digits indicates the
Unicode character with the specified "code point". For
example, \u05D0 represents a Hebrew letter Aleph. Here are
all codepoints for the Hebrew alphabet:
https://unicodeplus.com/block/0590 (for more info see:
https://developer.mozilla.org/en-US/docs/Glossary/Code_point)
- number: 123 and 123.45 are both valid numbers. Do not put quotes around
numbers. A number with quotes around it is considered a "string". Software
that processes the JSON will not associate a numeric value with numbers that
have "quotes" around them.
- boolean: boolean is just another way of saying "logical" data. A boolean
value can be either true or false. The words true/false must be lowercase.
Do not enclose the true and false in quotes. A true or false with quotes around
it is considered a "string".
- null: A value that is null is considered non-existant. For example, the
following JSON is used to represent some data about the ACME Shipping
Company. Notice that some employees have a middle name and some don't. The
ones who don't have a middle name have a value of null for the middle name.
{
"company": "ACME Shipping Company",
"employees": [ {"first": "Joe", "middle": null, "last": "Smith"},
{"first": "Sheila", "middle": null, "last": "Cohen"},
{"first": "Jack", "middle": "Edgar", "last": "Jones"} ]
}
Note that another way to indicate that an employee doesn't have a
middle name is just to leave out the middle name for that employee.
For example, the following is also valid JSON:
{
"company": "ACME Shipping Company",
"employees": [ {"first": "Joe", "last": "Smith"},
{"first": "Sheila", "last": "Cohen"},
{"first": "Jack", "middle": "Edgar", "last": "Jones"} ]
}
The choice of whether to use null or to simply leave out the data is
a choice made by the people who design the data format. Either are
valid choices. As mentioned above, the exact "spacing" of the data does
not affect the validity of the JSON file. The spacing is very useful
though to make the file easier to read.15.2 COMPLEX DATA TYPES - “OBJECTS” and “ARRAYS”
“Objects” and “arrays” are two different “complex data types” that allow you to organize multiple data values in different ways.
15.2.1 OBJECTS
A JSON “object” is surrounded by a single set of {curly braces}. Inside the curly braces are zero or more “name”:value pairs (also known as “key”:value pairs).
For example, the following is a valid JSON object:
{
"recipeName": "Midnight Chocolate Cake",
"submittedBy": "Jack Sep 1, 2022 9:00am",
"caloriesPerServing": 450,
"dietMenu": false,
"prepTime": "2 hours 45 minutes"
}Notice a few things about this example:
The data for the “object” is surrounded by { curly braces }.
Each line inside the {curly braces} contains a “name”:value pair (AKA “key”:value pair).
Note that it is NOT required that each “name”:value pair be on a separate line but it is considered good practice to make the files more readable.
The “name” in each “name”:value pair (AKA the “key” in each “key”:value pair) is enclosed in quotes.
It’s not uncommon to see JSON files that incorrectly leave out the quotes around the names. This is wrong. While these files may sometimes still be able to be processed correctly by software that accounts for this mistake, you should always make sure to include the “quotes” around the “names” in JSON files that you create yourself.
Note that technically the “name” (AKA “key”) in a name value pair may contain spaces. However, including spaces in names can sometimes cause problems when using software to process a JSON file. Therefore it is recommended not to use spaces in names. The example above uses “camelCase” for the names to clearly identify each word in a multiWord name.
The value in each “name”:value pair may be any valid JSON datatype. The different “name”:value pairs in the example above use different data types. For example “Midnight Chocolate Cake” is a string, 450 is a number and false is a boolean value. Note that “45 minutes” is a string and not a number.
The “name”:value pairs are separated from each other with commas. Note that there is NO comma after the last “name”:value pair in the object (i.e. no comma after “dietMenu”: false). Extra commas and missing commas are a common source of errors that can be hard to find.
The same “name” may not appear twice in a single object. For example, the following is NOT valid!
{ “thing”: “ball”, “thing”: “cookie” } # THIS IS NOT VALID - DUPLICATE NAMES
15.3 NESTED OBJECTS
The values in “name”:value pairs (AKA “key”:value pairs) may be objects. Below, is a modified version of the recipe example shown above. Notice that in the following version of the JSON, the value for “prepTime” is now itself an object. An object inside an object is known as a “nested object”.
{
"recipeName": "Midnight Chocolate Cake",
"submittedBy": "Jack Sep 1, 2022 9:00am",
"caloriesPerServing": 450,
"dietMenu": false,
"prepTime": {
"hours": 2,
"minutes": 45
}
}Continuing with the same example … we added another “name”:value pair whose name is “submittedBy”. The value of “submittedBy” is itself a nested object.
{
"recipeName": "Midnight Chocolate Cake",
"submittedBy": {
"userName": "Jack",
"when": {
"date": "Sep 1, 2022",
"time": "9:00 am"
}
},
"caloriesPerServing": 450,
"dietMenu": false,
"prepTime": {
"hours": 2,
"minutes": 45
}
}Continuing once again with the same example … we can also make the values for “date” and “time” into nested objects:
{
"recipeName": "Midnight Chocolate Cake",
"submittedBy": {
"userName": "Jack",
"when": {
"date": {
"month": "Sep",
"day": 1,
"year": 2022
},
"time": {
"hour": 9,
"minute": 0,
"am": true
}
}
},
"caloriesPerServing": 450,
"dietMenu": false,
"prepTime": {
"hours": 2,
"minutes": 45
}
}Objects may be nested for many levels. Without proper indentation, nested objects (and arrays - see below) can become very difficult to read. Make sure that your JSON objects are indented properly. (Note that the value in a “name”:value pair may also be an array as explained in the next section). ARRAYS
JSON arrays combine a sequence of values between [square brackets]. The values in an array are separated from each other using commas. There is no comma after the last value. For example, the following is a valid JSON array that contains several strings:
["apple", "pear", "plum"]Note that the values in an array are not “named” as they are in objects (see above). The following JSON array contains numbers:
[3.14159, 2.7182, 1.6180, 186282]There is technically no requirement that all values in a JSON array must be the same datatype (i.e. a “homogeneous array”). JSON does allow for “heterogeneous arrays”, or arrays that contain values of different data types. These are not as common as homogenous arrays, however they are still considered valid JSON. For example, the following is a JSON array that contains values of different data types.
[ "apple" , 3.14159 , true , null , false , true, 92 , "abc"]15.4 Nested Arrays
In addition to the simple data types (string, number, boolean, null), JSON arrays may also contain nested arrays as well as objects. For example, the following is an array that contains nested arrays.
[ [92, 80, 70, 83, 98, 77, 86, 99, 25] , [ 96, 72 , 26, 100, 92] , [ 93, 82, 92, 85] ]The above example contains three different nested arrays. Often such arrays are arranged with each “inner” array (that is contained in the “outer” array) on a different line. For example, the above JSON is equivalent to the following:
[
[92, 80, 70, 83, 98, 77, 86, 99, 25],
[96, 72, 26, 100, 92],
[93, 82, 92, 85]
]
```json
## Arrays that contain objects
Arrays may also contain objects. For example, the following shows an array that
contains 3 objects. Notice how the indentation is arranged to clearly show the
structure of the array and the objects.
```json
[
{
"first": "Sue",
"last": "Smith"
},
{
"first": "Frank",
"last": "Jones"
},
{
"name": "Anne",
"title": "Applebaum"
}
]15.5 Objects that contain arrays
Just as arrays can contain objects, objects can contain arrays. The value of a “name”:value pair in an object may be an array (as well as an object). For example, the following is a JSON object that describes a company. Below, the array of objects from the last example is used as the value for the “employees” key in the outermost object.
{
"companyName": "Widgets R Us",
"founded": 2022,
"employees":
[
{
"first": "Sue",
"last": "Smith"
},
{
"first": "Frank",
"last": "Jones"
},
{
"name": "Anne",
"title": "Applebaum"
}
]
}15.6 Complex JSON Files
Very complex JSON files can be constructed from a deep nesting of arrays that contain objects and objects that contain arrays, etc. etc. etc. The outermost structure in the example above is an object. It contains information about a single compmany. If we want to capture information about many companies, we could create an array of different company objects as shown below.
[
{
"companyName": "Widgets R Us",
"founded": 2022,
"employees":
[
{
"first": "Sue",
"last": "Smith"
},
{
"first": "Frank",
"last": "Jones"
},
{
"first": "Anne",
"last": "Applebaum"
}
]
},
{
"companyName": "Pete's Pizza",
"founded": 2020,
"employees":
[
{
"first": "Pete",
"last": "Jones"
},
{
"first": "Paula",
"last": "Jones"
}
}
]