​

console.log(typeof 37); //'number'

console.log(typeof NaN); //'number'

console.log(typeof ''); //'string'

console.log(typeof (typeof 1)); //'string'

console.log(typeof true); //'boolean'

console.log(typeof null); //'object'

console.log(typeof function(){}); //'function'

​

​

console.log(a);//值: undefined

console.log(typeof(a));//"undefined"

var a = 100;

console.log(a);//值: 100

console.log(typeof(a));//"number"

​

//console.log(b);//顯示 not defined

​

//驚嘆號(!)之前有說過是個邏輯運算符，名稱為"Logic NOT"，用在布林值上具有反轉(Inverted)的功能，雙驚嘆號(!!)就等於"反轉再反轉"，等於轉回原本的布林值:

var aBool = true;

console.log(aBool);

​

var bBool = !aBool; //false

console.log(bBool);

​

var cBool = !!aBool; //true

console.log(cBool);

​

//雙驚嘆號(!!)並不單純是在這樣用的，它是為了要轉換一些可以形成布林值的情況值，列出如下:

//false: 0, -0, null, false, NaN, undefined, 空白字串('')

//true: 不是false的其他情況

var aBool = !!0; //false

console.log(aBool);

​

var bBool = !!'false'; //true

console.log(bBool);

​

var cBool = !!NaN; //false

console.log(cBool);

​

​