Delete duplicate elements with Array.filter

- 3 min read

Manipulating arrays is one of the most common tasks when developing an application. Arrays are basic data structures in any program, and removing duplicate elements from an array is a common scenario that you may encounter.

In Javascript, removing duplicate elements from an array can be done in various ways, such as using Set, Array.reduce, Array.forEach, and Array.filter methods. In this article, we’ll focus on using the Array.filter method to remove duplicate elements from a Javascript array.

The Array.filter is an immutable method that returns a new array with the elements that pass the condition implemented by the callback function used as an argument. It iterates over the elements of the array and applies the callback function to each item, returning a boolean value. If the element passes the condition, true is returned, indicating that this element will be added to the new array.

To remove duplicate elements from an array using Array.filter, we can use the Array.indexOf method as a helper function. The Array.indexOf method returns the first index of the array where a given element is found. We can utilize this fact to check whether the element already exists in the new array that we want to create. If the element does not exist in the new array, we add it to the accumulator array.


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Here’s an example of how to use Array.filter to remove duplicate elements from a Javascript array:

const data = [1, 2, 6, 1, 2, 5, 9, '33', '33'];

const result = data.filter((item, index) => {
  return data.indexOf(item) === index;

console.log(result); // [1, 2, 6, 5, 9, '33']


In this example, we define a new constant called data that contains an array of values, some of which are duplicated. We then use the Array.filter method and pass a callback function that takes two arguments: the current value item and the current index index. In the callback function, we use the Array.indexOf method to check whether the current value item is the first occurrence in the array. If it is, the index returned by the Array.indexOf method will be equal to the index argument passed to the callback function. Therefore, we can safely assume that we are looking at the first occurrence of the item value in the array, and we can add it to the new array that we want to create.

Finally, we log the result array to the console, which should contain only the unique elements from the original data array.

In conclusion, using Array.filter is a simple and effective way of removing duplicate elements from a Javascript array. By taking advantage of the Array.indexOf method, we can easily check whether an element already exists in the new array that we want to create and add it only if it’s not already present.

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