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 you want to check if a variable (say v) has been defined and is not null:

if (typeof v !== 'undefined' && v !== null) {
    // Do some operation  
}

If you want to check for all falsy values such as: undefined, null, '', 0, false:

if (v) {
   // Do some operation
}