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

问题

I'm wallowing in ES2015+ luxury with a few projects right now and am wondering whether I can get rid of the much hated crutch to check for undefined in the new wonderland.

Is there a shorter but still exact way to typeof varName === 'undefined' in ES2015+ already?

Of course I could use default parameters but this also feels like an unnecessary assignment.

function coolFn(a = null){
   if (a===null) console.log("no a supplied");
}

回答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.

回答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).

来源：https://stackoverflow.com/questions/34596489/es2015-2016-way-of-typeof-varname-undefined