JSON: objects and arrays

JSON is names and values

JSON writes a record as names and values. Curly brackets hold a thing with named parts. Square brackets hold a list. Those two shapes cover almost everything.

A form, as JSON

Here is the same made up person form. Turn it over, and this is JSON. The curly brackets make an object, a set of named values. Each name is a string in quotes, then a colon, then its value. A comma separates one pair from the next. And the square brackets make an array, an ordered list. Here the person has two email addresses, in order.

Six characters, two structures

JSON has just six structural characters. Left and right curly brackets, around an object. Left and right square brackets, around an array. A colon between a name and its value. And a comma between items. With four simple types, strings, numbers, true or false, and null, and two structures, objects and arrays, that is the whole format.

• Simple types · String, number, true or false, null. · tag
• Array · [ ] an ordered list of values. · layers
• Object · { } named values; order not defined. · box
• Separators · : between name and value; , between items. · link

Object and array

The standard defines the two structures carefully. An object is an unordered collection of name and value pairs. An array is an ordered sequence of values. The names within an object should be unique. If they are not, the standard says software behaviour is unpredictable, and some software reports only the last value.

Check yourself

Pause and think. Which is ordered in JSON: an object or an array? The answer: an array.

• An array
• An object
• Both are ordered
• Neither is ordered

Keep these in mind

An object holds named values in curly brackets. An array holds an ordered list in square brackets. Names are strings in quotes, and should be unique within an object. And there are four simple types: string, number, true or false, and null.

• An object holds named values in curly brackets
• An array holds an ordered list in square brackets
• Names are strings in quotes, unique within an object
• Four simple types: string, number, true or false, null