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

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);

For filtering and return only unique names, you can use Array#filter with Set.

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 = family.filter((set => f => !set.has(f.name) && set.add(f.name))(new Set));

console.log(unique);

The Solution

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 occurrences = {}

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

you can also generalize this solution to a function

function filterByProperty(array, propertyName) {
  var occurrences = {}

  return array.filter(function(x) {
    var property = x[propertyName]
    if (occurrences[property]) {
      return false;
    }
    occurrences[property]] = true;
    return true;
  })
}

and use it like

var filteredFamily = filterByProperty(family, 'name')

Explanation

Don't 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. 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). Thus, it's better to use an object, which has O(1) lookup.

This will work fine.

const result = [1, 2, 2, 3, 3, 3, 3].reduce((x, y) => x.includes(y) ? x : [...x, y], []);

console.log(result);