Boolean

代表兩個值之一：true或false

需要注意的是 "true"(字串) true(布林值) 這兩個是不一樣的東西

一元運算符Unary operator 「!」可以將Boolean的值反轉

console.log(!true);  // -> false
console.log(!false);  // -> true

typeof 可以用來確認資料類型

console.log(typeof true);  // -> boolean

Undefined

已經宣告變數，卻沒有賦予initializer時，就會出現undefined

undefined也是JavaScript中的functions的預設return value

let x;
console.log(x);  // -> Undefined (x is waiting for assignment，現在尚未放東西進去)

Null

代表某個故意不存在的值

let y = null;
console.log(y);  // -> null (y has nothing inside，刻意宣告為空值)

運算符 Operators

兩個運算元 operand 與一個運算符 operator 可以算出一個數字

運算符中，常見的有：

• 賦值 assignment operator
• 比較 comparison operator
• 邏輯 logical operator
• 屬性 typeof operator (unary)
• 否定 negation operator (unary) -> !
• 遞增 increment operator (unary) -> x++
• 遞減 Decrement operator (unary) -> x--
• 位元 bitwise operator
• 算術 arithmetic operator

comparison operator

運算元 operand 是兩個任意資料型態，且運算結果為Boolean 值

operand (number, string, boolean) , result (boolean)

• == return true if the operands are equal

  console.log(3 == 3);  // -> true
  console.log(3 == 6);  // -> false, comparison operator
  console.log(3 = 6);   // ->        assignment operator
  

• != return true if the operands are not equal

• === return true if the operands are equal and of the same data type

  console.log(3 == "3");  // -> true， == 代表去檢查運算元的值
  console.log(3 === "3"); // -> false ， === 代表去檢查運算元的值，也檢查data type
  

• !== return true if the operands are of the same type but not equal, or are of different type

  console.log(3 !== "3");  // -> true
  

• > return true if the left operand is greater than the right operand

• >= return true if the left operand is greater than or equal to the right operand

• < return true if the left operand is less than the right operand

• <= return true if the left operand is less than or equal to the right operand

logical operator

兩個任意資料型態，且運算結果為 Boolean

// comparison & logical 結合
console.log(5 > 3 && 100 > 99);  // -> true

下一篇會來學習位元運算子。