ES2015/2016 way of 'typeof varName === 'undefined`?

Solution 1:

Just check for varName === undefined.

In older browsers it was possible to assign an alternate value to the global undefined variable causing that test to fail, but in ES2015+ that's now impossible.

Note that there's no way to distinguish explicitly passing undefined as a parameter from leaving the parameter out altogether other than by looking at arguments.length.

Solution 2:

The only case where typeof varName === 'undefined' is useful is when you don't know whether the variable varName has been declared.

And IMO if you don't know whether your variables are declared, your code has serious problems.

In other cases you have better options:

• varName === void 0

  This will detect whether varName is undefined.

  void is an operator which receives an argument (you can use whatever instead of 0), and returns undefined.

• varName === undefined

  This should detect whether varName is undefined.

  However, be aware the global undefined could have been overwritten (before ES5) or shadowed with another value. Therefore I prefer void, which is also shorter.

• varName == null

  This will detect whether varName is undefined or is null.

• !varName

  This will detect whether varName is falsy (undefined, null, 0, empty string, NaN, false).