You may have read that JavaScript’s % operator is a remainder operator, not a modulo operator. This blog post explains what that means.

Originally published at https://2ality.com

Let’s assume there are two operators that are very similar:

- The
*remainder operator*`rem`

- The
*modulo operator*`mod`

If both operands have the same signs, then the operators produce the same results:

> 5 rem 3 2 > 5 mod 3 2> -5 rem -3 -2 > -5 mod -3 -2

If, however, they have different signs, then the result of `rem`

has the same sign as the first operand, while the result of `mod`

has the same sign as the second operand:

> -5 rem 3 -2 > -5 mod 3 1> 5 rem -3 2 > 5 mod -3 -1

Why is that? Read on.

`rem`

We’ll first take a closer look at the `rem`

operator:

dividend rem divisor

In order to compute the remainder, we use the following two equations:

dividend = divisor * quotient + remainder |remainder| < |divisor|

`dividend`

, `divisor`

, `quotient`

, and `remainder`

are all integers (for the sake of this blog post).

In order to compute the remainder we must find the `quotient`

:

remainder = dividend - divisor * quotient

And we do so by dividing the `dividend`

by the `divisor`

. `Math.trunc()`

ensures that the resulting `quotient`

is an integer.

**Example 1:** `5 rem 3 === 2`

const dividend = 5; const divisor = 3; const quotient = Math.trunc(dividend / divisor); assert.equal(quotient, 1);

**Example 2:** `-5 rem 3 === -2`

const dividend = -5; const divisor = 3; const quotient = Math.trunc(dividend / divisor); assert.equal(quotient, -1);

`mod`

The modulo operator is based on the same equations, but it uses `Math.floor()`

to compute quotients:

- If both dividend and divisor are positive, the modulo operator produces the same results as the remainder operator (example 3).
- If, however, dividend and divisor have different signs, then the results are different (example 4).

The reason for that is that `Math.trunc()`

and `Math.floor()`

produce the same results for positive numbers, but different results for negative numbers.

**Example 3:** `5 mod 3 === 2`

(dividend is 5, divisor is 3)

const dividend = 5; const divisor = 3; const quotient = Math.floor(dividend / divisor); assert.equal(quotient, 1);

**Example 4:** `-5 mod 3 === 1`

(dividend is −5, divisor is 3)

const dividend = -5; const divisor = 3; const quotient = Math.floor(dividend / divisor); assert.equal(quotient, -2);

Modulo can also be viewed as an operation that maps an arbitrary number into a given range – for example:

x mod 3

maps `x`

into the range

[0,3) = {0,1,2}

That is, zero is included, 3 is excluded.

If `x`

is already inside the range, performing the mapping is simple:

> 0 mod 3 0 > 2 mod 3 2

If `x`

is greater than or equal to the upper boundary of the range, then the upper boundary is subtracted from `x`

until it fits into the range:

> 4 mod 3 1 > 7 mod 3 1

That means we are getting the following mapping for non-negative integers:

0 1 2 3 4 5 6 7 0 1 2 0 1 2 0 1

This is how the mapping is extended so that it also covers negative integers:

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 2 0 1 2 0 1 2 0 1 2 0 1 2 0 1 > -1 mod 3 2 > -3 mod 3 0 > -4 mod 3 2

`x rem 3`

maps `x`

as follows:

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 -1 0 -2 -1 0 -2 -1 0 1 2 0 1 2 0 1

`x mod -3`

maps `x`

to the range `(-3, 0]`

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 -1 0 -2 -1 0 -2 -1 0 -2 -1 0 -2 -1 0 -2

`x rem -3`

has the following mapping (the same as `x rem 3`

):

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 -1 0 -2 -1 0 -2 -1 0 1 2 0 1 2 0 1

The ECMAScript specification uses modulo several times – for example:

· To convert the operands of the `>>>`

operator to unsigned 32-bit integers (via `x mod 2`

**32****):**

· > 232 >>> 0 · 0 · > (232)+1 >>> 0 · 1 · > (-1 >>> 0) === (232)-1 · true

· To convert arbitrary numbers so that they fit into Typed Arrays. For example, `x mod 2**8`

is used to convert numbers to unsigned 8-bit integers (after first converting them to integers):

· const tarr = new Uint8Array(1); · · tarr[0] = 256; · assert.equal(tarr[0], 0); · · tarr[0] = 257; · assert.equal(tarr[0], 1);

Which of `rem`

and `mod`

is supported and how depends on the programming language:

- JavaScript‘s
`%`

operator is a remainder operator. - Python’s
`%`

operator is a modulo operator.

If you want to read more about remainder and modulo:

- The Wikipedia page on “modulo operation” has much information.
- The ECMAScript specification mentions that
`%`

is computed via “truncating division”. - Sect. “Rounding” in “JavaScript for impatient programmers” describes several ways of rounding in JavaScript.

**Thanks for reading** ❤

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