how to build a javascript object using an array and the map function?

Question

I've got an array of channels that I want to transform into a single object (channelSettings) with a true / false property for each channel .

I've got it working using the below code but it seems verbose. Is there are way to do it without the "temp" var? If I can get ride of that, then I could get ride of the self executing function as well.

var channels = ["TV", "Billboard", "Spot TV"];


var channelSettings = function() {
    var temp = {};

    channels.map(function(itm, i, a) {
        var channel = itm.toLowerCase().replace(" ", "");
        temp[channel] = false;
    });

    return temp;
}();

I guess I'm trying to get the map function to return an object with properties instead of an array. Is this possible? Is it mis-guided? Suggestions?

This is what I'm hoping it looks like in the end:

var channels = ["TV", "Billboard", "Spot TV"];

var channelSettings = channels.map(function(itm, i, a) {
        var channel = itm.toLowerCase().replace(" ", "");
        return ????;
});

Answer 1

Use a .reduce() function instead.

var channelSettings = channels.reduce(function(obj, itm) {
        var channel = itm.toLowerCase().replace(" ", "");
        obj[channel] = false;

        return obj;
}, {});

DEMO: http://jsfiddle.net/MjW9T/

---

The first parameter references the previously returned item, except for the first iteration, where it references either the first item in the Array, or the seeded item, which we provided as an empty object.

The second parameter references the current value in the Array. As long as we always return obj , the first parameter will always be that object, as will the final return value.

Answer 2

The map function takes an array, and returns an array. Nothing else. But you can use reduce :

var settings = ["TV", "Billboard", "Spot TV"].reduce(function(obj, item) {
   obj[item.toLowerCase().replace(" ", "")] = false; // probably too concise
   return obj // yay, we can skip a semi-colon here :-P
}, {});

Well, "am not i am" beat me to it, but anyway:
map not only returns arrays, but also only returns arrays of the same length as the original. It's meant for transforming one array's values 1:1 into a new array. reduce is meant to "reduce an array to a single value". Hence its use here.

If you use a straight for loop or the forEach method to add properties to an object, you do need to declare that object. So, no, you can't do without temp in your code (unless you use reduce instead of a loop).

More information on MDN:

1. map: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/map
2. reduce: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/Reduce

Answer 3

hmm.. looks like wrapping it in a function like this would do it.

function toObject(arr) {
  var rv = {};
  for (var i = 0; i < arr.length; ++i)
    if (arr[i] !== undefined) rv[arr[i]] = true;
  return rv;
}