Restricts a number n to the interval 0 <= n < m by "wrapping it around" within said range.

const wrap = require('wrap-around')
var result = {}
for (var m = 3, n = -3; n < 9; n++) {
  result[n] = wrap(m, n)
}

Some possible uses of this function include:

• restricting an index to valid array indices

index = wrap(array.length, index)

• selecting items from the end of a list (Python-style!)

> var array = ['foo', 'bar', 'baz']
> array[wrap(array.length, -1)]
'baz'

• wrapping the player around the screen in a game of Pac-Man, Snake, etc.

hero.position = [wrap(width, x), wrap(height, y)]

While the modulo operator wraps positive values with ease (it's actually used internally by the wrap function), it takes a bit more setup to handle negative values correctly. Consider the following example, in which % fails to provide the desired result:

> -1 % 3
-1

> wrap(3, -1)
2

Using loops for this kind of thing is a handy way of demonstrating what exactly this function does - wrap(m, n) produces the same result as two loops forcing a number between the desired range.

while (n < 0) n += m
while (n >= m) n -= m

Unfortunately, they're also 300x slower. 😬

# wrap 100000 times
ok ~4.7 ms (0 s + 4696167 ns)

# loop 100000 times
ok ~1.36 s (1 s + 359910028 ns)

So at the end of the day, you're better off avoid loops for "wrapping" numbers in production code. Use the modulo % operator for positive numbers or wrap-around if you plan on handling negative numbers.

MIT © Brandon Semilla