[javascript] How can I check if the array of objects have duplicate property values?

I need some help with iterating through array, I keep getting stuck or reinventing the wheel.

values = [
    { name: 'someName1' },
    { name: 'someName2' },
    { name: 'someName1' },
    { name: 'someName1' }
]

How could I check if there are two (or more) same name value in array? I do not need a counter, just setting some variable if array values are not unique. Have in mind that array length is dynamic, also array values.

if you are looking for a boolean, the quickest way would be

var values = [_x000D_
    { name: 'someName1' },_x000D_
    { name: 'someName2' },_x000D_
    { name: 'someName1' },_x000D_
    { name: 'someName1' }_x000D_
]_x000D_
_x000D_
// solution_x000D_
var hasDuplicate = false;_x000D_
values.map(v => v.name).sort().sort((a, b) => {_x000D_
  if (a === b) hasDuplicate = true_x000D_
})_x000D_
console.log('hasDuplicate', hasDuplicate)

//checking duplicate elements in an array
var arr=[1,3,4,6,8,9,1,3,4,7];
var hp=new Map();
console.log(arr.sort());
var freq=0;
for(var i=1;i<arr.length;i++){
// console.log(arr[i-1]+" "+arr[i]);
if(arr[i]==arr[i-1]){
freq++;

}
else{
hp.set(arr[i-1],freq+1);
freq=0;
}
}
console.log(hp);

To know if simple array has duplicates we can compare first and last indexes of the same value:

The function:

var hasDupsSimple = function(array) {

    return array.some(function(value) {                            // .some will break as soon as duplicate found (no need to itterate over all array)
       return array.indexOf(value) !== array.lastIndexOf(value);   // comparing first and last indexes of the same value
    })
}

Tests:

hasDupsSimple([1,2,3,4,2,7])
// => true

hasDupsSimple([1,2,3,4,8,7])
// => false

hasDupsSimple([1,"hello",3,"bye","hello",7])
// => true

For an array of objects we need to convert the objects values to a simple array first:

Converting array of objects to the simple array with map:

var hasDupsObjects = function(array) {

  return array.map(function(value) {
    return value.suit + value.rank

  }).some(function(value, index, array) { 
       return array.indexOf(value) !== array.lastIndexOf(value);  
     })
}

Tests:

var cardHand = [
  { "suit":"spades", "rank":"ten" },
  { "suit":"diamonds", "rank":"ace" },
  { "suit":"hearts", "rank":"ten" },
  { "suit":"clubs", "rank":"two" },
  { "suit":"spades", "rank":"three" },
]

hasDupsObjects(cardHand);
// => false

var cardHand2 = [
  { "suit":"spades", "rank":"ten" },
  { "suit":"diamonds", "rank":"ace" },
  { "suit":"hearts", "rank":"ten" },
  { "suit":"clubs", "rank":"two" },
  { "suit":"spades", "rank":"ten" },
]

hasDupsObjects(cardHand2);
// => true

In TS and ES6 you can create a new Set with the property to be unique and compare it's size to the original array.

const values = [_x000D_
  { name: 'someName1' },_x000D_
  { name: 'someName2' },_x000D_
  { name: 'someName3' },_x000D_
  { name: 'someName1' }_x000D_
]_x000D_
_x000D_
const uniqueValues = new Set(values.map(v => v.name));_x000D_
_x000D_
if (uniqueValues.size < values.length) {_x000D_
  console.log('duplicates found')_x000D_
}

With Underscore.js A few ways with Underscore can be done. Here is one of them. Checking if the array is already unique.

function isNameUnique(values){
    return _.uniq(values, function(v){ return v.name }).length == values.length
}

With vanilla JavaScript By checking if there is no recurring names in the array.

function isNameUnique(values){
    var names = values.map(function(v){ return v.name });
    return !names.some(function(v){ 
        return names.filter(function(w){ return w==v }).length>1 
    });
}

ECMA Script 6 Version

If you are in an environment which supports ECMA Script 6's Set, then you can use Array.prototype.some and a Set object, like this

let seen = new Set();
var hasDuplicates = values.some(function(currentObject) {
    return seen.size === seen.add(currentObject.name).size;
});

Here, we insert each and every object's name into the Set and we check if the size before and after adding are the same. This works because Set.size returns a number based on unique data (set only adds entries if the data is unique). If/when you have duplicate names, the size won't increase (because the data won't be unique) which means that we would have already seen the current name and it will return true.

ECMA Script 5 Version

If you don't have Set support, then you can use a normal JavaScript object itself, like this

var seen = {};
var hasDuplicates = values.some(function(currentObject) {

    if (seen.hasOwnProperty(currentObject.name)) {
        // Current name is already seen
        return true;
    }

    // Current name is being seen for the first time
    return (seen[currentObject.name] = false);
});

The same can be written succinctly, like this

var seen = {};
var hasDuplicates = values.some(function (currentObject) {
    return seen.hasOwnProperty(currentObject.name)
        || (seen[currentObject.name] = false);
});

Note: In both the cases, we use Array.prototype.some because it will short-circuit. The moment it gets a truthy value from the function, it will return true immediately, it will not process rest of the elements.

Try an simple loop:

var repeat = [], tmp, i = 0;

while(i < values.length){
  repeat.indexOf(tmp = values[i++].name) > -1 ? values.pop(i--) : repeat.push(tmp)
}

Demo

You can use map to return just the name, and then use this forEach trick to check if it exists at least twice:

var areAnyDuplicates = false;

values.map(function(obj) {
    return obj.name;
}).forEach(function (element, index, arr) {
    if (arr.indexOf(element) !== index) {
        areAnyDuplicates = true;
    }
});

Fiddle