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up vote 0 down vote favorite

Given a table represented as a javascript array of objects, I would like create a list of unique values for a specific property.

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

var groups = _.groupBy(table, "a");
var array = [];
_.forOwn(groups, function(value, key){
  array.push(key);
});

console.log(array);
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.1/lodash.min.js"></script>
Check your console

The problems with this code is that the groupBy keep track of the values, which I really don't care about, and I need to iterate over the entire list generated by group again so I can yank out the keys. It seems pretty inefficient to me and Im wondering if there is a smarter way to do this without reinventing the wheel (or perhaps if I should just bite the bullet and reinvent the wheel).

share|improve this question
up vote 1 down vote

You can always use reduce. Lodash/underscore has a similar function, if compatibility is an important matter.

function getUniqueValuesOfKey(array, key){
  return array.reduce(function(carry, item){
    if(item[key] && !~carry.indexOf(item[key])) carry.push(item[key]);
    return carry;
  }, []);
}

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

// See snippet result
document.write(JSON.stringify(getUniqueValuesOfKey(table, 'a')));

share|improve this answer
up vote 0 down vote

I'm not sure exactly what you're looking for, but here's an implementation in plain Javascript:

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

function getUniqueValues(array, key) {
    var result = new Set();
    array.forEach(function(item) {
        if (item.hasOwnProperty(key)) {
            result.add(item[key]);
        }
    });
    return result;
}

log('getUniqueValues(table, "a")');
getUniqueValues(table, "a").forEach(log);

log('getUniqueValues(table, "b")');
getUniqueValues(table, "b").forEach(log);

// utility function to display results in snippet
function log(value) {
    document.write(value + "<br>");
}

share|improve this answer
up vote 0 down vote

You could also chain map and filter:

function unique(arr, prop) {
    return arr.map(function(e) { return e[prop]; }).filter(function(e,i,a){
        return i === a.indexOf(e);
    });
}

console.log(unique(table,'a')); //[1,2]
console.log(unique(table,'b')); //[2,3,4]
share|improve this answer
up vote 0 down vote

I found out there is an even simpler way to do this. The _.countBy() method does exactly what I want:

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


var array = _.countBy(table, function(t){
  return t.a;
});

console.log(array);
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.1/lodash.min.js"></script>
Check your console

share|improve this answer

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