app.js

// Hello. I'm learning. Thank you for sharing your insights.
var sUtil = require('sUtils.js');

var obj = {
        a:[1,2],
        b:[3,4],
}

// (1) Check if is array. If it is array, then apply function to it's value.

// Imperative
if (Array.isArray(obj.a)) {  = sUtil.bitmaskArrayToDecimal(obj.a); }
if (Array.isArray(obj.b)) {  = sUtil.bitmaskArrayToDecimal(obj.b); }
        
// Functional style BUT it's wrong and fails to work :(
[ obj.a, obj.b ].forEach(function(o){
  if (Array.isArray(o)) {
        o = sUtil.bitmaskArrayToDecimal(o); // The value doesn't get attributed to source 'obj'.
  }
});

// If I wanted a new array i'd use .map() but what I need is to change previous data. "o =" doesn't cut it!
// How can I fix the more 'functional' code?

sUtils.js

var bitmaskArrayToDecimal = export = function (arr) {
	var result = 0;
	for (var i = 0; i < arr.length; i++) {
		result = result | (1 << arr[i] - 1);
	};
	return result;
}