Variables and data types

• Explain what variables are
• Use variables
• Explain the basic data types available in JavaScript

See the Mozilla Developer Network article on grammar and types.

See W3schools.com JavaScript variables article

• Variables are named memory locations that can contain values.
• The var keyword can (and should) be used to declare variables in JavaScript.
• The var keyword helps control the scope of a variable (local to the function it is declared in)
• The let keyword helps control the scope of a variable (local to the code block it is declared in) [recently added]
• The const keyword helps control the scope of a variable (local to the code block it is declared in), and also makes the variable a constant [recently added]
• Strict mode requires variables to be declared using the "var" keyword
• Variables do NOT have to be declared before they are used in JavaScript. The first use of a variable will automatically declare it if it isn't already declared - but don't do that. It can cause problems.
• You can set the value of a variable using an assignment operator (=).
• Naming variables:
  • The first character of the name must be a letter, '$', or underscore
  • Subsequent characters must be letters, digits, underscore, or '$'
  • Names are case sensitive
  • You can't use words reserved by the JavaScript language
• Variables in JavaScript don't have a set data type (such as String or Number). They can change data types as needed. This is not uncommon in scripting languages.
• Example of declaring a variable with function scope: var num;
• Example of declaring a variable with block scope: let num;
• Example of declaring multiple variables with one statement: var num, x;
• Example of setting a variable's data type when declaring it: var num = new Number;
• Example of setting a variable to a value: num = 7;
• Example of declaring and initializing a variable in one statement: var num = 7;
• Example of declaring and initializing a constant: const PI = 3.1415926;
• Example of creating and initializing multiple variables in one statement: var num = 7, greeting = "Hi";
• Example of using a variable: var numSquared = num * num; // numSquared will be 49 (assuming num is 7)

See the Mozilla Developer Network article on JavaScript data types.

See W3schools.com JavaScript data types article.

• Primitive types
  • Boolean: true or false
  • Null: you can set a variable to null to get rid of its value
  • Undefined: the value of a variable which contains no value
  • Number: numbers can be written with or without a decimal point; exponential representation can also be used (5e3 is 5000, 5e-3 is .005)
  • String: a series of characters; string literals are contained within quotes; the quotes can be either single or double quotes
  • Symbol: new in ES6; symbols are ubnique, immutable primitives that can be used as a key for a property
• Non-primitive type
  • Object: a collection of properties and their values; the properties can include functions

Note: Arrays, Functions, Dates, etc. are objects.