JavaScript Numbers

Numbers

JavaScript has one number type. This stores both integers and floating point numbers (numbers with digits after the decimal point)

let score = 42
const total = 10.1
const pi = 3.14159265

Basic Math

+, -, *, /

+ and - represent addition and subtraction.

* and / represent multiplication and division.

Example:

const firstNumber = 42
const secondNumber = 4
const result = firstNumber / secondNumber

In this case result is 10.5.

%

This is known as the REMAINDER operator. The result of firstNumber % secondNumber is equal to whatever the remainder would be if we divided secondNumber into firstNumber.

Taking our above example:

const firstNumber = 42
const secondNumber = 4
const result = firstNumber / secondNumber
const remainder = firstNumber % secondNumber

In this case result is 10.5 and remainder is 2. The remainder is 2 because as we divide 4 into 42 we can only do so 10 times, leaving 2 over.

CAUTION: Remainder with negative numbers!

const firstNumber = -42
const secondNumber = 4
const result = firstNumber / secondNumber
const remainder = firstNumber % secondNumber

In this case, result is -10.5 and remainder is -2! NOTE, in some languages the % means a modulus and will always be positive

Remainder/% is useful for things like determining if a number is even or odd.

const isThreeEven = 3 % 2 == 0 // False
const isEightEven = 8 % 2 == 0 // True

It is also good for ensuring that a value is bounded by some value, causing it to wrap around.

// We don't want index to be 5 or more (stops at 4), and if it does it should wrap around to 0
let index = 0
index = (index + 1) % 5 // 0 + 1 is 1 -- 1 % 5 = 1
console.log(`index is ${index}`)
index = (index + 1) % 5 // 1 + 1 is 2 -- 2 % 5 = 2
console.log(`index is ${index}`)
index = (index + 1) % 5 // 2 + 1 is 3 -- 3 % 5 = 3
console.log(`index is ${index}`)
index = (index + 1) % 5 // 3 + 1 is 4 -- 4 % 5 = 4
console.log(`index is ${index}`)
// Here is where the % comes into play, adding one to 4 gives 5, but we want this to "wrap around" back to 0.
index = (index + 1) % 5 // 4 + 1 is 5 -- 5 % 5 = 0
console.log(`index is ${index}`)
index = (index + 1) % 5 // 0 + 1 is 1 -- 1 % 5 = 1
console.log(`index is ${index}`)
index = (index + 1) % 5 // 1 + 1 is 2 -- 2 % 5 = 2
console.log(`index is ${index}`)

Let's see this in a loop:

for (let index = 0; index < 20; index++) {
  const wrappedValue = index % 5
  console.log(`Wrapped value is ${wrappedValue} since index is ${index}`)
}

Wrapped value is 0 since index is 0
Wrapped value is 1 since index is 1
Wrapped value is 2 since index is 2
Wrapped value is 3 since index is 3
Wrapped value is 4 since index is 4
Wrapped value is 0 since index is 5
Wrapped value is 1 since index is 6
Wrapped value is 2 since index is 7
Wrapped value is 3 since index is 8
Wrapped value is 4 since index is 9
Wrapped value is 0 since index is 10
Wrapped value is 1 since index is 11
Wrapped value is 2 since index is 12
Wrapped value is 3 since index is 13
Wrapped value is 4 since index is 14
Wrapped value is 0 since index is 15
Wrapped value is 1 since index is 16
Wrapped value is 2 since index is 17
Wrapped value is 3 since index is 18
Wrapped value is 4 since index is 19

Conversions

parseInt('42')

Attempts to convert the given string into an int value. It does its best effort to parse what it is given. If it can't figure out how to parse the number we get back the special value NaN which represents "Not A Number". This isn't a string, but a special value, in the same way null is a special value.

Example:

const answer = parseInt('42')
// answer is 42
const mostlyAnAnswer = parseInt('42 things')
// mostlyAnAnswer is still 42
const notAnAnswer = parseInt('I think the answer is 42')
// notAnAnswer is the value NaN

Number('42')

Another way to convert a string to a number is with the Number method. Number is more strict than parseInt in that the string must be strictly formatted as a number. Any illegal characters will result in a value of NaN.

const answer = Number('42.5')
// answer is 42.5
const mostlyAnAnswer = Number('42 things')
// mostlyAnAnswer is the value NaN
const notAnAnswer = Number('I think the answer is 42')
// notAnAnswer is the value NaN

ToString

A variable storing a number can use the toString method to create a string representation.

Example

const value = -16325
const valueAsString = value.toString()
// valueAsString will be `-16325`

Rounding

JavaScript offers a number of ways to round numbers. The primary rounding methods are round, ceiling, and floor.

floor

The floor method, Math.floor, accepts a number and returns the number without any of the digits after the decimal part. The technical description is Returns the largest integral value less than or equal to the specified number.

const price = 12.34
const priceFloored = Math.floor(price)
// priceFloored will be 12

Ceiling

The ceiling method, Math.ceil, accepts a number and returns the smallest whole number that is greater than or equal to the number. For instance if we asked for the ceiling of 42 we would get 42 back since it 42 a whole number that is equal to the number we gave it. However, if we supplied 42.01, the next smallest whole number would be 43.

const wholePrice = 42
const wholePriceCeiling = Math.ceil(wholePrice)
// wholePriceCeiling will be 42
const priceWithMore = 42.01
const priceWithMoreCeiling = Math.ceil(priceWithMore)
// priceWithMoreCeiling will be 43

Round

Rounds a value to the nearest whole number. Optionally we can specify the number of fractional digits.

const pi = 3.14159265
const roundedPi = Math.round(pi)
// roundedPi will be 3 since we are rounding down