JS中各种变量类型在条件判断为false的情况

var a = undefined; ->false
var a = 0; ->false
var a = 0.0; ->false
var a = NaN; ->false
var a = ""; ->false
var a = null; ->false
var a = false; ->false
var a ->false
function a(){ };

//调用无返回值的函数，默认返回值是undefined

a() ->false