Generating an array of unique values of a specific property in an object array

Question

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).

Answer 1

I know this is an old post but purely for those looking for ways to do this heres a way that I found works nicely with a single line

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

let result = [...new Set(table.map(item => item.a))];
document.write(JSON.stringify(result));

Answer 2

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')));

Answer 3

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]

Answer 4

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>");
}

Answer 5

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

Answer 6

This might not be the best optimised code but definitely works!!

      let flag= 0;
      for(let i=0; i<Arr1.length; i++) {
        if(arr2[i]['prop']) {
           flag =0;
          for(let j=0; j<Arr1.length; j++){
            if(Arr1[j]['prop']=== arr2[i]['prop']){
              flag = 1;
              break;
            }
          }
          if(!flag) {
            Arr1.push(arr2[i]);
          }
        }
      }