Remainder Modulus
suggest changeThe remainder / modulus operator (%
) returns the remainder after (integer) division.
console.log( 42 % 10); // 2
console.log( 42 % -10); // 2
console.log(-42 % 10); // -2
console.log(-42 % -10); // -2
console.log(-40 % 10); // -0
console.log( 40 % 10); // 0
This operator returns the remainder left over when one operand is divided by a second operand. When the first operand is a negative value, the return value will always be negative, and vice versa for positive values.
In the example above, 10
can be subtracted four times from 42
before there is not enough left to subtract again without it changing sign. The remainder is thus: 42 - 4 * 10 = 2
.
The remainder operator may be useful for the following problems:
- Test if an integer is (not) divisible by another number:
x % 4 == 0 // true if x is divisible by 4 x % 2 == 0 // true if x is even number x % 2 != 0 // true if x is odd number
Since
0 === -0
, this also works forx <= -0
. - Implement cyclic increment/decrement of value within
[0, n)
interval.Suppose that we need to increment integer value from
0
to (but not including)n
, so the next value aftern-1
become0
. This can be done by such pseudocode:var n = ...; // given n var i = 0; function inc() { i = (i + 1) % n; } while (true) { inc(); // update something with i }
Now generalize the above problem and suppose that we need to allow to both increment and decrement that value from
0
to (not including)n
, so the next value aftern-1
become0
and the previous value before0
becomen-1
.var n = ...; // given n var i = 0; function delta(d) { // d - any signed integer i = (i + d + n) % n; // we add n to (i+d) to ensure the sum is positive }
Now we can call
delta()
function passing any integer, both positive and negative, as delta parameter.
Using modulus to obtain the fractional part of a number
var myNum = 10 / 4; // 2.5
var fraction = myNum % 1; // 0.5
myNum = -20 / 7; // -2.857142857142857
fraction = myNum % 1; // -0.857142857142857