Find duplicate property values in an Array of Objects in JavaScript

Written by Raymon Schouwenaar on

Finding duplicate property values in an Array of Objects in JavaScript is not always that simple if you thought, so let’s dive in! A few days ago I needed to work this out, so I thought let’s write it down, so I could use it later again!

Array of Objects

Let’s define an example Array of Objects, where each Object is a user:

var myArray = [
  {id: 1, name: 'Foo Bar', email: ''},
  {id: 2, name: 'Bar Foo', email: ''},
  {id: 3, name: 'Joe Ocean', email: ''},
  {id: 3, name: 'Jenny Block', email: ''},

You can see that there are a few property value’s that are duplicated. The id and the email property. But if you can imagine in a real application you don’t know if someone else has used an email address before. So you need to check it before you can save it.

I’m not gonna discuss a process how you should do this in an application. But just showing how you could see if an Array of Objects has duplicated property values.

JavaScript function to check duplicate property values

function checkDuplicateInObject(propertyName, inputArray) {
  var seenDuplicate = false,
      testObject = {}; {
    var itemPropertyName = item[propertyName];
    if (itemPropertyName in testObject) {
      testObject[itemPropertyName].duplicate = true;
      item.duplicate = true;
      seenDuplicate = true;
    else {
      testObject[itemPropertyName] = item;
      delete item.duplicate;

  return seenDuplicate;

Let’s create a function with 2 parameters. I will explain everything from the function, so everyone could follow this :-).

The propertyName you want to check in each Object and the inputArray, where you are gonna check the Objects from.

We define 2 variables at the start. seenDuplicate, the value is gonna be false as default. After that, we create a testObject to put in every Object we are looping trough.

We use the .map() function to loop trough the Array (read the article .map() vs .forEach() vs .for() to know why I picked .map()). For every Object we check if there is an property in the testObject with the name of the id, email or something else. Because the in keyword checks the property name not the value of a property!

If there isn’t a property name in the testObject that is the same as the item’s property value, then it will be placed in the Object. If it is already in the testObject it will be marked as duplicated. But more important, seenDuplicate will be true!

After the .map() function we return the seenDuplicate variable. So the result of the function, if we check the id property will be true. If we check it with the property name it will be false. Or with email, it will return true!


In the demo below you can test the output of the console.

JS Bin on{.jsbin-embed}

If you have some question’s or comments on this post, please add them to the comments 🙂

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