5 * 2
// 5 = right operand
// 2 = left operand
// operand sometimes call arguments
// An operator is unary
let x = 1;
x = -x;
alert( x ); // -1, unary negation was applied
// An operator is binary
let x = 1, y = 3;
alert( y - x ); // 2, binary minus subtracts values
String concatenation, binary +
+ 若有一邊的值是字串會轉換成字串相加。
alert(2 + 2 + '1' ); // "41" and not "221"
Numeric conversion, unary +
unary + 把值轉換成數字
// Converts non-numbers does the same thing as Number(...)
alert( +true ); // 1
alert( +"" ); // 0
如果從 HTML 得到值通常是字串，可以用 + 轉換成數字。
let apples = "2";
let oranges = "3";
// both values converted to numbers before the binary plus
// unary + precedance > binary precedance
alert( +apples + +oranges ); // 5
// the longer variant
// alert( Number(apples) + Number(oranges) ); // 5
Operator precedence
Assignment
= 也是運算符號，通常返回一個值，並由右到左執行。
let a, b, c;
a = b = c = 2 + 2;
alert( a ); // 4
alert( b ); // 4
alert( c ); // 4
Remainder %
計算餘數
alert( 5 % 2 ); // 1 is a remainder of 5 divided by 2
alert( 8 % 3 ); // 2 is a remainder of 8 divided by 3
alert( 6 % 3 ); // 0 is a remainder of 6 divided by 3
Exponentiation **
計算次方
alert( 2 ** 2 ); // 4 (2 * 2)
alert( 4 ** (1/2) ); // 2 (power of 1/2 is the same as a square root, that's maths)
Increment / decrement
let counter = 2;
counter++; // works the same as counter = counter + 1, but is shorter
alert( counter ); // 3
let counter = 2;
counter--; // works the same as counter = counter - 1, but is shorter
alert( counter ); // 1
alert( 5++ ); // error only applied to variables
在過程中使用前綴或後綴沒有差別，但結果包含前綴跟後綴是有差別的。
let counter = 0;
counter++;
++counter;
alert( counter ); // 2, the lines above did the same
let counter = 0;
alert( ++counter ); // 1 prefix
let counter = 0;
alert( counter++ ); // 0 postfix
Bitwise operators
AND ( & )
OR ( | )
XOR ( ^ )
NOT ( ~ )
LEFT SHIFT ( << )
RIGHT SHIFT ( >> )
ZERO-FILL RIGHT SHIFT ( >>> )
Modify-in-place
想要對變數做運算後，儲存新的值在同一變數可以使用縮寫。
let n = 2;
n += 5; // now n = 7 (same as n = n + 5)
n *= 2; // now n = 14 (same as n = n * 2)
alert( n ); // 14
Comma
最奇怪的運算符號，可以運算多個運算式，用，分開，但只有最後一個值會被返回。
let a = (1 + 2, 3 + 4);
alert( a ); // 7 (the result of 3 + 4)
// three operations in one line
for (a = 1, b = 3, c = a * b; a < 10; a++) {
...
}