Checking if a variable exists in javascript

Question

I know there are two methods to determine if a variable exists and not null(false, empty) in javascript:

1) if ( typeof variableName !== \'undefined\' && v

Answer 1

if (variable) can be used if variable is guaranteed to be an object, or if false, 0, etc. are considered "default" values (hence equivalent to undefined or null).

typeof variable == 'undefined' can be used in cases where a specified null has a distinct meaning to an uninitialised variable or property. This check will not throw and error is variable is not declared.