算術運算子

算術運算子接受數值（資料型態為字串或變數都可）作為其運算子並回傳單個數值。 標準算術運算符號有加號（+），減號（ - ），乘（*）和除（/）。

加法運算子生成數字(運算元)的總和或字串串接。

Operator: x + y

// Number + Number -> addition
1 + 2 // 3

// Boolean + Number -> addition
true + 1 // 2

// Boolean + Boolean -> addition
false + false // 0

// Number + String -> concatenation
5 + 'foo' // "5foo"

// String + Boolean -> concatenation
'foo' + false // "foofalse"

// String + String -> concatenation
'foo' + 'bar' // "foobar"

減法運算子能算出兩個運算元間的差異。

Operator: x - y

5 - 3 // 2
3 - 5 // -2
'foo' - 3 // NaN

The division operator produces the quotient of its operands where the left operand is the dividend and the right operand is the divisor.

Operator: x / y

1 / 2      // returns 0.5 in JavaScript
1 / 2      // returns 0 in Java
// (neither number is explicitly a floating point number)

1.0 / 2.0  // returns 0.5 in both JavaScript and Java

2.0 / 0    // returns Infinity in JavaScript
2.0 / 0.0  // returns Infinity too
2.0 / -0.0 // returns -Infinity in JavaScript

The multiplication operator produces the product of the operands.

Operator: x * y

2 * 2 // 4
-2 * 2 // -4
Infinity * 0 // NaN
Infinity * Infinity // Infinity
'foo' * 2 // NaN

The remainder operator returns the remainder left over when one operand is divided by a second operand. It always takes the sign of the dividend.

Operator: var1 % var2

12 % 5 // 2
-1 % 2 // -1
1 % -2 // 1
NaN % 2 // NaN
1 % 2 // 1
2 % 3 // 2
-4 % 2 // -0
5.5 % 2 // 1.5

The exponentiation operator returns the result of raising first operand to the power second operand. that is, var1var2, in the preceding statement, where var1 and var2 are variables. Exponentiation operator is right associative. a ** b ** c is equal to a ** (b ** c).

Operator: var1 ** var2

In most languages like PHP and Python and others that have an exponentiation operator (**), the exponentiation operator is defined to have a higher precedence than unary operators such as unary + and unary -, but there are a few exceptions. For example, in Bash the ** operator is defined to have a lower precedence than unary operators. In JavaScript, it is impossible to write an ambiguous exponentiation expression, i.e. you cannot put a unary operator (+/-/~/!/delete/void/typeof) immediately before the base number.

-2 ** 2;
// 4 in Bash, -4 in other languages.
// This is invalid in JavaScript, as the operation is ambiguous.


-(2 ** 2);
// -4 in JavaScript and the author's intention is unambiguous.

2 ** 3 // 8
3 ** 2 // 9
3 ** 2.5 // 15.588457268119896
10 ** -1 // 0.1
NaN ** 2 // NaN

2 ** 3 ** 2 // 512
2 ** (3 ** 2) // 512
(2 ** 3) ** 2 // 64

To invert the sign of the result of an exponentiation expression:

-(2 ** 2) // -4

To force the base of an exponentiation expression to be a negative number:

(-2) ** 2 // 4

The increment operator increments (adds one to) its operand and returns a value.

• If used postfix, with operator after operand (for example, x++), then it returns the value before incrementing.
• If used prefix with operator before operand (for example, ++x), then it returns the value after incrementing.

Operator: x++ or ++x

// Postfix
var x = 3;
y = x++; // y = 3, x = 4

// Prefix
var a = 2;
b = ++a; // a = 3, b = 3

The decrement operator decrements (subtracts one from) its operand and returns a value.

• If used postfix (for example, x--), then it returns the value before decrementing.
• If used prefix (for example, --x), then it returns the value after decrementing.

Operator: x-- or --x

// Postfix
var x = 3;
y = x--; // y = 3, x = 2

// Prefix
var a = 2;
b = --a; // a = 1, b = 1

The unary negation operator precedes its operand and negates it.

Operator: -x

var x = 3;
y = -x; // y = -3, x = 3

// Unary negation operator can convert non-numbers into a number
var x = "4";
y = -x; // y = -4

The unary plus operator precedes its operand and evaluates to its operand but attempts to convert it into a number, if it isn't already. Although unary negation (-) also can convert non-numbers, unary plus is the fastest and preferred way of converting something into a number, because it does not perform any other operations on the number. It can convert string representations of integers and floats, as well as the non-string values true, false, and null. Integers in both decimal and hexadecimal ("0x"-prefixed) formats are supported. Negative numbers are supported (though not for hex). If it cannot parse a particular value, it will evaluate to NaN.

Operator: +x

+3     // 3
+'3'   // 3
+true  // 1
+false // 0
+null  // 0
+function(val){ return val } // NaN

• Assignment operators