[javascript] not not (!!) 運算

!!false	===	false
!!true	===	true
!!0	===	false
!!parseInt("foo")	===	false	// NaN is falsy
!!1	===	true
!!-1	===	true	// -1 is truthy
!!""	===	false	// empty string is falsy
!!"foo"	===	true	// non-empty string is truthy
!!"false"	===	true	// ...even if it contains a falsy value
!!window.foo	===	false	// undefined is falsy
!!null	===	false	// null is falsy
!!{}	===	true	// an (empty) object is truthy
!![]	===	true	// an (empty) array is truthy; PHP programmers beware!