# The Modulus Operator in JavaScript

The remainder operator, returns the remainder when the first
operand is divided by the second operand. It is also sometimes called the *modulus operator*, although technically the modulus operator is a different concept.

A handy analogy for the remainder operator is buying things: if widgets cost $3 and you have $7,
you can buy at most 2 widgets. Then you'll have $1 remaining. The remainder operator
`a % b`

returns what's left if you buy as many widgets as possible if you have `a`

dollars and widgets cost `b`

dollars.

`7 % 3; // 1`

In English, `a % b`

is often spoken as "a mod b".

The remainder operator has several neat uses:

## Is A Number Odd or Even?

An integer `n`

is even if it is
divisible by 2. The remainder operator returns 0 if `a`

is divisible by `b`

.
In other words, checking if a number is even is equivalent to checking if `n % 2 === 0`

.

```
function isEven(n) {
return n % 2 === 0;
}
isEven(5); // false
isEven(7.6); // false
isEven(8); // true
```

Similarly, `n`

is odd if and only if `n % 2 === 1`

.

```
function isOdd(n) {
return n % 2 === 0;
}
isEven(5); // false
isEven(7.6); // false
isEven(8); // true
```

## The Fractional Part of a Number

Say you have a number `2.5`

and you want to get just the part of the number
after the decimal point `0.5`

. The most concise way to do this is to take
the number mod 1.

```
function getFractionalPart(n) {
return n % 1;
}
getFractionalPart(2.5); // 0.5
```

## Converting Minutes to Hours

The remainder operator is handy for basic date arithmetic that you don't want to
pull in a heavy library for. For example, if you're given a number `n`

that represents
a number of minutes (potentially more than 60) and you want to convert it to hours
and minutes, you could do something like this:

```
const minutesToHoursAndMinutes = n => ({
hours: Math.floor(n / 60),
minutes: n % 60
});
minutesToHoursAndMinutes(59); // { hours: 0, minutes: 59 }
minutesToHoursAndMinutes(62); // { hours: 1, minutes: 2 }
minutesToHoursAndMinutes(123); // { hours: 2, minutes: 3 }
```

## Remainder vs Modulus

Modulus is a related concept, but handles negative numbers differently. For example, `-21 % 5 === -1`

, because the remainder always takes the sign of the left number. However, a true modulus operator would always return a positive value,
so 21 modulo 5 would equal 4. In practice, you are unlikely to use the remainder
operator on negative values, and many JavaScript developers aren't aware of the
difference.

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