You can just check if the variable has a truthy value or not. That means

if( value ) {
}

will evaluate to true if value is not :

  • null
  • undefined
  • NaN
  • empty string ("")
  • 0
  • false

The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section.

Furthermore, if you do not know whether a variable exists (that means, if it was declared ) you should check with the typeof operator. For instance

if( typeof foo !== 'undefined' ) {
    // foo could get resolved and it's defined
}

If you can be sure that a variable is declared at least, you should directly check if it has a truthy value like shown above.

Further read: http://typeofnan.blogspot.com/2011/01/typeof-is-fast.html