JavaScript check if variable exists (is defined/initialized)

Question

Which method of checking if a variable has been initialized is better/correct? (Assuming the variable could hold anything (string, int, object, function, etc.))

Answer 1

I use two different ways depending on the object.

if( !variable ){
  // variable is either
  // 1. '';
  // 2. 0;
  // 3. undefined;
  // 4. null;
  // 5. false;
}

Sometimes I do not want to evaluate an empty string as falsey, so then I use this case

function invalid( item ){
  return (item === undefined || item === null);
}

if( invalid( variable )){
  // only here if null or undefined;
}

If you need the opposite, then in the first instance !variable becomes !!variable, and in the invalid function === become != and the function names changes to notInvalid.