Select unique elements from an Array using: lastIndexOf() & indexOf()

Obieze Ezeugo Felistus — Sat, 27 Aug 2022 23:25:00 +0000

Hello guys! So, today I am going to show you how to use the following built-in array methods (lastIndexOf() & indexOf()) to select unique elements from an array.

indexOf()

The indexOf() method of the array returns the first index or position at which a given element can be found in the array. It returns -1 if the element is not found.
Eg:

const numberArray = [1,2,3,4];
const index = numberArray.indexOf(3);
console.log(index) => 2

const noIndex = numberArray.indexOf(9);
console.log(noIndex) => -1

lastIndexOf()

The lastIndexOf() method of the array returns the last index or position at which an element can be found in an array. The array is searched backwards.
Eg:

const numberArray = [1,2,3,4,5,7,3,6,9];
const lastIndex = numberArray.lastIndexOf(3);
console.log(lastIndex ) => 6

const noLastIndex = numberArray.lastIndexOf(90);
console.log(noLastIndex ) => -1

Now, let's go into the business of the day.
NOTE: The purpose of this tutorial is not to make an array of repeated values a unique array, rather it is to pick elements that appear only once in a repeated array.

Example

Given an array of repeated values:

const repeatedVals = [5, 7, 3, 9, 4, 9, 8, 3, 2, 7, 2];
We are going to output a new array of unique values.

function uniqueValues(arr) {
  const uniqueVals = [];

  return uniqueVals
}

Explanation: we created a function, inside the function, we created an empty array to hold the unique values and then return that new array. Next, we are going to write our for-loop and some conditions.

// [5, 7, 3, 9, 4, 9, 8, 3, 2, 7, 2];

function uniqueValues(arr) {
  const uniqueVals = [];
  const lenOfArr = arr.length;

  for (let i = 0; i < lenOfArr; i++){
    const val = arr[i];
    if (arr.lastIndexOf(val) === arr.indexOf(val)) {
      uniqueVals.push(val)
    }
  }
  return uniqueVals
}
uniqueValues(repeatedVals)

Output-

[5, 4, 8]

Explanation:
The if conditional simply checks if the returned indices of both methods are equal. In the case where they are not equal, it simply means that the current value has a duplicate in the array.

Thank you for checking out this post. In case of any errors or suggestions, kindly notify me.

DEV Community: Obieze Ezeugo Felistus

Select unique elements from an Array using: lastIndexOf() & indexOf()

indexOf()

lastIndexOf()

Example

Output-