How to find duplicate values in a JavaScript array of objects, and output only unique values?

I would probably set up some kind of object. Since you've said ECMAScript 6, you have access to Set, but since you want to compare values on your objects, it will take a little more work than that.

An example might look something like this (removed namespace pattern for clarity):

var setOfValues = new Set();
var items = [];

function add(item, valueGetter) {
   var value = valueGetter(item);
   if (setOfValues.has(value))
      return;

   setOfValues.add(value);
   items.push(item);
}

function addMany(items, valueGetter) {
   items.forEach(item => add(item, valueGetter));
}

Use it like this:

var family = [
...
];

addMany(family, item => item.name);

// items will now contain the unique items

Explanation: you need to pull a value from each object as it's added and decide if it has already been added yet, based on the value you get. It requires a value getter, which is a function that given an item, returns a value (item => item.name). Then, you only add items whose values haven't already been seen.

A class implementation:

// Prevents duplicate objects from being added

class ObjectSet {
  constructor(key) {
    this.key = key;
    this.items = [];
    this.set = new Set();
  }

  add(item) {
    if (this.set.has(item[this.key])) return;
    this.set.add(item[this.key]);
    this.items.push(item);
  }

  addMany(items) {
    items.forEach(item => this.add(item));
  }
}

var mySet = new ObjectSet('name');
mySet.addMany(family);
console.log(mySet.items);

The solution here is to not compare objects using indexOf, which only uses the === operator between objects. The reason why your current answer doesn't work is because === in JS does not compare the objects deeply, but instead compares the references. What I mean by that you can see in the following code:

var a = { x: 1 }
var b = { x: 1 }

console.log(a === b) // false
console.log(a === a) // true

Equality will tell you if you found the same exact object, but not if you found an object with the same contents.

In this case, you can compare your object on name since it should be a unique key. So obj.name === obj.name instead of obj === obj.

So store occurrences of name external to the loop in an object, and filter if there's been a previous occurrence.

https://jsfiddle.net/nputptbb/2/

var occ = {}

var filtered = family.filter(function(x) {
  if (occ[x.name]) return false;
  occ[x.name] = true;
  return true;
})

Moreover another problem with your code that affects its runtime and not its function is that you use an indexOf inside of your reduce. indexOf is O(n), which makes the complexity of your algorithm O(n^2).

It's better to use an object, which has O(1) lookup.

You could use a Set in combination with Array#map and a spread operator ... in a single line.

Map returns an array with all names, which are going into the set initializer and then all values of the set are returned in an array.

var family = [{ name: "Mike", age: 10 }, { name: "Matt", age: 13 }, { name: "Nancy", age: 15 }, { name: "Adam", age: 22 }, { name: "Jenny", age: 85 }, { name: "Nancy", age: 2 }, { name: "Carl", age: 40 }],
    unique = [...new Set(family.map(a => a.name))];

console.log(unique);

With the code you mentioned, you can try:

family.filter((item, index, array) => {
  return array.map((mapItem) => mapItem['name']).indexOf(item['name']) === index
})

Or you can have a generic function to make it work for other array of objects as well:

function printUniqueResults (arrayOfObj, key) {
  return arrayOfObj.filter((item, index, array) => {
    return array.map((mapItem) => mapItem[key]).indexOf(item[key]) === index
  })
}

and then just use printUniqueResults(family, 'name')

(FIDDLE)