@polesye

What would be wrong with:

var substractDelta = function (value, side) {
    if (_.isNumber(value) && _.isNumber(delta[side])) {
	delta[side] -= value;
  }

   return module;
}

Same number of lines, one function call less, and 'value' argument checked for validity before any numerical operation is done on it (like value = -1 * value;)