Truthy and Falsy

Values that evaluate to true

1

'0';

2	'any string';

3	[];  // an empty array

4	{};  // an empty object

5

1;   // any non-zero number

Values that evaluate to false

1

0;

2	'';  // an empty string

3	NaN; // JavaScript's "not-a-number" variable

4

null;

5	undefined;  // be careful -- undefined can be redefined!

JQuery - Referencing the core jQuery file within the document header

Go to http://code.google.com/apis/libraries/devguide.html where the jQuery code file can be piped

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.0/jquery.min.js"></script>