Declaring variable using var after assignment to the variable [duplicate]

Question

Possible Duplicate:
is there any Advantage of redeclaring javascript variables?

Why does the following code display 1 rather than undefined:

a = 1;
var a;
alert(a);

Answer 1

1) var does not redeclare or delete a variable

2) even if it did, your code gets rewritten* using javascript hoisting rules (any variable or function declaration gets moved to the top of the nearest enclosing function) as follows:

var a;
a = 1;
alert(a);

(*effectively rewritten; see RobG's comments and links on entering of execution contexts for clarification)