How to Check if the Array of Objects Have Duplicate Property Values with JavaScript? | by John Au-Yeung | Mar, 2024

Sometimes, we want to check if the array of objects have duplicate property values with JavaScript.

In this article, we’ll look at how to check if the array of objects have duplicate property values with JavaScript.

To check if the array of objects have duplicate property values with JavaScript, we can use the JavaScript array’s map and some method to map the array to the property values we want to check.

Then we can use the indexOf method in the some callback to check if an element in the mapped array is the first instance or not.

For instance, we can write:

const values = [{
name: 'someName1'
},
{
name: 'someName2'
},
{
name: 'someName4'
},
{
name: 'someName2'
}
];const valueArr = values.map((item) => {
return item.name
});
const isDuplicate = valueArr.some((item, idx) => {
return valueArr.indexOf(item) !== idx
});
console.log(isDuplicate);

We have the values array that we want to check if there’re duplicate values of the name property.

Then we call map to return an array with the name property values and assign it to valueArr .

Next, we call valueArr.some with a callback that checks if idx is different from the index returned by indexOf .

Since indexOf returns the index of the first instance of a value, that means if idx is different from what’s returned with indexOf , we know it’s a duplicate value.

Therefore, since we have a duplicate value of name in 2 entries, isDuplicate is true and that’s logged in the console log.

To check if the array of objects have duplicate property values with JavaScript, we can use the JavaScript array’s map and some method to map the array to the property values we want to check.

Then we can use the indexOf method in the some callback to check if an element in the mapped array is the first instance or not.