The JavaScript Arithmetic operators

Math shows up in almost every program. JavaScript gives you a familiar set of arithmetic operators.

• Addition (+)
• Subtraction (-)
• Division (/)
• Remainder (%)
• Multiplication (*)
• Exponentiation (**)
• Increment (++)
• Decrement (--)
• Unary negation (-)
• Unary plus (+)

Addition (+)

const three = 1 + 2
const four = three + 1

+ also concatenates strings. Mix types and you get concatenation, not addition:

const three = 1 + 2
three + 1 // 4
'three' + 1 // three1

Subtraction (-)

const two = 4 - 2

Division (/)

Returns the quotient of the first operand and the second:

const result = 20 / 5 //result === 4
const result = 20 / 7 //result === 2.857142857142857

Divide by zero and JavaScript returns Infinity (or -Infinity for negative numerators):

1 / 0 //Infinity
-1 / 0 //-Infinity

Remainder (%)

The remainder is handy for wrapping indices and checking even numbers:

const result = 20 % 5 //result === 0
const result = 20 % 7 //result === 6

Remainder by zero is NaN:

;(1 % 0) - //NaN
  (1 % 0) //NaN

Multiplication (*)

Multiply two numbers:

1 * 2 - //2
  1 * 2 //-2

Exponentiation (**)

Raise the first operand to the power of the second:

1 ** 2 //1
2 ** 1 //2
2 ** 2 //4
2 ** 8 //256
8 ** 2 //64

** matches Math.pow():

Math.pow(4, 2) == 4 ** 2

Increment (++)

Increment a number. Postfix returns the old value, then increments. Prefix increments first, then returns the new value:

let x = 0
x++ //0
x //1
++x //2

Decrement (--)

Works like increment, but subtracts one:

let x = 0
x-- //0
x //-1
--x //-2

Unary negation (-)

Flip the sign:

let x = 2 - x //-2
x //2

Unary plus (+)

Coerce a value to a number when it is not already one:

let x = 2 + x //2

x = '2' + x //2

x = '2a' + x //NaN

Operator precedence follows school math: multiplication and division before addition, unless parentheses group terms. When in doubt, add parentheses. Future you will thank present you.

The modulo operator is how I check even numbers: n % 2 === 0. It is also how I wrap an index inside a fixed range.

Try 10 % 3, '5' + 2, and +'42' in the console. Predict each result before you run it. You should see 1, '52', and 42.