Part 8/258 Number#mod

For some bizarre reason, JavaScript does not constrain the result of the % operator to be between 0 and the base. This can make computing the index into an array or “wrapping” a negative number a small pain. This method gives all numbers an improved mod method that will guarantee they end up inside the array.

/**
 * A mod method useful for array wrapping. The range of the function is
 * constrained to remain in bounds of array indices.
 *
 * Example:
 * (-1).mod(5) === 4
 *
 * @param {Number} base
 * @returns An integer between 0 and (base - 1) if base is positive.
 * @type Number
 */
Number.prototype.mod = function(base) {
  var result = this % base;
 
  if(result < 0 && base > 0) {
    result += base;
  }
 
  return result;
};

This makes jumping into an index much easier.

// Example usage
var result = array[n.mod(array.length)];

Though still not quite as nice as in Ruby where you can do:

result = array[n] # Ruby cares enough to make it happen

I love Ruby…