Variable Declarations

Variables

var VAR-NAME;
var VAR-NAME = VALUE;
var VAR-NAME, VAR-NAME;

VAR-NAME = VALUE

• Variables are declared using the keyword var or by assigning a value to them.
• var is optional for global variables but required for local variables (inside a function).
• IE <= 7 may require that global variables be explicitly declared with var.
• If a variable is redeclared, it will still have its original value.

• Global variables and functions are members of the global object.
• In browsers, the global object contains a window member whose value is the global object.

• There are no explicit data types.
• There are implicit data types, based on the context in which the variable is used.
• JavaScript often tries to treat all variables in a statement as if they had the same type as the first variable in the statement.

• Variable names must start with a letter or underscore.
• A variable can optionally be initialized (assigned a value) when it is declared.
• Multiple variables can be declared on the same line by separating their names with commas.

• Evaluating an unassigned (uninitialized) variable

• Results in the undefined value, or NaN in number contexts, if the variable was declared with var.
• Results in a runtime error if the variable was declared without var.

var x;
var y = 10;
var a, b;
i = 1;

Constants

const x = 1;

• A constant declared within a function is local; otherwise, it's global in scope
• Supposedly, IE <= 7 does not support constants