Get object keys for filtered values

Question

The case is simple - I\'ve got a following object:

Object {1: false, 2: true, 3: false, 4: false, 5: false, 6: false, 7: false, 8: true, 12: false, 13: false, 14         


        

Answer 1

var data = {1: false, 2: true, 3: false, 4: true};

var filteredIds = _.filter(_.keys(data), function (key) {
    return data[key];
});

// result [2, 4]


var rejectedIds = _.reject(_.keys(data), function (key) {
    return data[key];
});

// result [1, 3]

Answer 2

var obj = {1: false, 2: true /*...*/};

you can use reduce:

_(obj).reduce(function(memo, val, key){
    if (val)
        memo.push(key);
    return memo;
}, []);

or chain map:

_(obj).chain().map(function(val, key){
    if (val)
        return key;
}).reject(_.isUndefined).value();

Answer 3

You can use _.pick. Like this:

var data = {1: false, 2: true, 3: false, 4: false, 5: false, 6: false, 7: false, 8: true, 12: false, 13: false, 14: false, 15: false, 16: false, 17: false, 18: false, 19: false}

var keys = _.keys(_.pick(data, function(value) {
  return value;
}));

Answer 4

var keys = [];
_.each( obj, function( val, key ) {
  if ( val ) {
    keys.push(key);
  }
});

There may be easier/shorter ways using plain Underscore.

---

In case anyone here uses Lodash instead of Underscore, the following is also possible, which is very short and easy to read:

var keys = _.invert(obj, true)[ "true" ];

Answer 5

You can do it very easily

var obj = { a: false, b: false, c: true, d: false, e: true, f: false };
    name, names;

name = _.findKey(obj, true);
// -> 'c'
names = _.findKeys(obj, true);
// -> ['c', 'e']

For that you juste need to extend underscore a little :

_.mixin({

  findKey: function(obj, search, context) {
    var result,
        isFunction = _.isFunction(search);

    _.any(obj, function (value, key) {
      var match = isFunction ? search.call(context, value, key, obj) : (value === search);
      if (match) {
        result = key;
        return true;
      }
    });

    return result;
  },

  findKeys: function(obj, search, context) {
    var result = [],
        isFunction = _.isFunction(search);

    _.each(obj, function (value, key) {
      var match = isFunction ? search.call(context, value, key, obj) : (value === search);
      if (match) {
        result.push(key);
      }
    });

    return result;
  }
});

And you can even use a function as filter, like this :

var team = {
  place1: { name: 'john', age: 15 },
  place2: { name: 'tim', age: 21 },
  place3: { name: 'jamie', age: 31 },
  place4: { name: 'dave', age: 17 }}

// Determine the places of players who are major
var placeNames = _.findKeys(team, function(value) { return value.age >= 18; });
// -> ['place2', 'place3']

Enjoy ;-)

Answer 6

Giorgi Kandelaki gave a good answer, but it had some potential problems (see my comment on his answer).

The correct way is:

_(obj).pairs().filter(_.last).map(_.first)

or

_.map(_.filter(_.pairs(obj),_.last),_.first)