var a = 123;
console.log( isNaN( a ) );
//控制檯列印false
//isNaN(num)方法為判斷num是否為NaN,如果是則返回true,否者返回false

var a = NaN;
console.log( isNaN( a ) );
//控制檯列印為true

var a = undefined;
console.log( isNaN( a ) );
//控制檯列印為true;undefined明顯不等於NaN,那麼為啥結果為true了？

因為isNaN(num)判斷時先將num隱式轉化Number(num)，在於NaN比較

var a = undefined;
//Number(undefined),結果為NaN
console.log( isNaN( a ) );
//控制檯列印為true
 

var a = "aaa";
//Number(a),結果為NaN
console.log( isNaN( a ) );
//控制檯列印為true

var a = "";
//Number(a),結果為0
console.log( isNaN( a ) );
//控制檯列印為false

var a = true;
//Number(a),結果為1
console.log( isNaN( a ) );
//控制檯列印為false

var a = "123ad";
//Number(a),結果為NaN
console.log( isNaN( a ) );
//控制檯列印為true

var a = "123";
//Number(a),結果為123
console.log( isNaN( a ) );
//控制檯列印為false
 

var a = window;
//Number(a),結果為NaN
console.log( isNaN( a ) );
//控制檯列印為true

var a = null;
//Number(a),結果為0
console.log( isNaN( a ) );
//控制檯列印為false

var a = Object;
//Number(a),結果為NaN
console.log( isNaN( a ) );
//控制檯列印為true

var a = [1,2,3];
//Number(a),結果為NaN
console.log( isNaN( a ) );
//控制檯列印為true