Javascript Null check

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 don't 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 truthyvalue like shown above.