2/ Note that we can create our own special “parameter is missing” values via default parameter values (=we don’t really need a built-in `undefined`):
const MISSING = Symbol('MISSING');
function f(param = MISSING) {
return param === MISSING ? 'MISSING' : param;
}
Using this function:
> f()
'MISSING'
> f(undefined) // also triggers the default value
'MISSING'
> f(null)
null
> f('abc')
'abc'