Data Types
Every programming language has its own built-in data types. The data types in JavaScript include numbers, strings, booleans, arrays, and objects.
Number
In JavaScript, numbers can be whole (such as 10) or contain a decimal (such as 10.5).
String
Strings store text, such as letters, words, symbols, and spaces. Strings are surrounded by quotation marks.
Boolean
A boolean stores the value true or false.
Array
An array is a list containing multiple items. The items can be different data types: numbers, strings, and even other arrays. An array inside another array is called a nested array.
Array Indexing
Array indexing provides a way to access a specific item in an array. In most programming languages, including JavaScript, array indices start at 0. For example, the first item in an array is at index 0, the second item is at index 1, etc.
Object
An object stores an unordered list of properties. Each property is a key:value pair. The key stores the property's name. The value can be any data type, such as a number, string, array, or even another object.
Object Dot Notation
Dot notation is one way to access a property of an object. To use dot notation, write the name of the object, followed by a dot (.), followed by the name of the property.
Object Bracket Notation
Bracket notation is another way to access a property of an object. To use bracket notation, write the name of the object, followed by brackets []. Inside the brackets, write the property name as a string.
Undefined
When a variable has not been assigned a value, its value is undefined.