Ultimate string parsing map reduce action.

parser.js

var string = 'colors:red, colors:blue, sizes:42, sizes:44';

var obj =  string.split(', ')
  .map(function(entry) {
    return entry.split(':').reduce(function(prev, curr) {
      if (!prev.name) {
        prev.name = curr;
        prev.value = '';
      } else {
        prev.value += curr;
      }

      return prev;
    }, {});
  })
  .reduce(function(prev, curr) {
    if (!prev.hasOwnProperty(curr.name)) {
      prev[curr.name] = [curr.value];
    }
    if (!~prev[curr.name].indexOf(curr.value)) {
      prev[curr.name].push(curr.value);
    }

    return prev;
  }, {});

// { colors: ['red', 'blue'], sizes: [42, 44] }
console.log(obj);