javascript - undefined and NaN, convert to boolean value implicitly vs explicitly -

i'm learning javascript following book "you dont know js".

in section "type & grammer", when discussing implicit vs explicit boolean convertion, author mentioned

//come function make sure 1 argument truthy  //implicit convertion function onlyone() {   var sum = 0;   (var i=0; < arguments.length; i++) {       // skip falsy values. same treating       // them 0's, avoids nan's.       if (arguments[i]) {           sum += arguments[i];       }   }   return sum == 1; }  //explicit convertion function onlyone() {   var sum = 0;   (var i=0; < arguments.length; i++) {       sum += number( !!arguments[i] );   }   return sum === 1; } 

is explicit coercion form of utility "better"? avoid nan trap explained in code comments. but, ultimately, depends on needs. think former version, relying on implicit coercion more elegant (if won't passing undefined or nan), , explicit version needlessly more verbose.

my question is, nan trap author talking about? thought when undefined , nan converted boolean value, regardless of whether converted implicitly or explicitly, both results in false. , passing undefined , nan implicit function ok, right?

i think example of real explicit check be...

function onlyone() {   var sum = 0;   (var i=0; < arguments.length; i++) {       sum += boolean( arguments[i] );   }   return sum == 1; } 

this of course avoid / guard against nan , should return false if no arguments present; if no arguments truthy , of course -if more 1 arguments truthy.