Concatenate Nested Array after Mapping

Another method using straight Javascript:

var x = categories.map(function(val) {
  return val.items;
}).reduce(function(pre, cur) {
   return pre.concat(cur);
}).map(function(e,i) {
  return {label:e.name,value:e.itemId};
});

Output: x = [{label: "Item1", value: 1}, {label: "Item2", value: 2}, …]

The concat() method is used to join two or more arrays.

This method does not change the existing arrays, but returns a new array, containing the values of the joined arrays.

http://jsfiddle.net/vymJv/1/

for(var i = 0; i < categories.length; i++){
    items = items.concat($.map(categories[i].items, function(elem) {
        return {value: elem.itemId, label: elem.name};
    }));
}