JavaScript Program to Find the First Non-Repeated Element in an Array

Finding the first non-repeated element in an array refers to identifying the initial occurrence of an element that does not occur again elsewhere within the array, indicating uniqueness among the elements.

Examples:

Input: {-1, 2, -1, 3, 0}
Output: 2
Explanation: The first number that does not repeat is : 2
Input: {9, 4, 9, 6, 7, 4}
Output: 6
Explanation: The first number that does not repeat is : 6

We have common approaches to perform this:

Approach 1 : Using For Loop in JavaScript

In this approach, we are using a nested for loop, iterate through each element in an array. Compare it with every other element to find the first non-repeated element. If found, print it; otherwise, indicate that all elements are repeated.

Syntax:

for (let i in obj1) {
    // Prints all the keys in
    // obj1 on the console
    console.log(i);
}

Example: Below is the implementation of the above approach using nesting For loop.

let arr = [9, 4, 9, 6, 7, 4];
let n = arr.length;
let nonRepeatedFound = false;

for (let i = 0; i < n; i++) {
    let j;
    for (j = 0; j < n; j++) {
        if (i != j && arr[i] == arr[j]) {
            break;
        }
    }
    if (j == n) {
        console.log(
            "The first non-repeated element is:",
            arr[i]
        );
        nonRepeatedFound = true;
        break;
    }
}
if (!nonRepeatedFound) {
    console.log(
        "All elements in the array are repeated."
    );
}

The first non-repeated element is: 6

Approach 2: Using find() Method

In this approach, we use the find method, search for the first non-repeated element in an array. then we use a ternary operator to display either the element or a message indicating that all elements are repeated.

Syntax:

array.find(function(currentValue, index, arr),thisValue);

Example: Below is the implementation of the above approach using Find method.

let arr = [9, 4, 9, 6, 7, 4];

let nonRepeated = arr.find(
    (num) =>
        arr.indexOf(num) === arr.lastIndexOf(num)
);

let result =
    nonRepeated !== undefined
        ? "The first non-repeated element is: " +
          nonRepeated
        : "All elements in the array are repeated.";

console.log(result);

The first non-repeated element is: 6

Approach 3: Using map() Method

In this approach, map() method is used to iterate through an array, updating the count of each element using a Map, and then finding the first non-repeated element from the modified data.

Syntax:

map((element, index, array) => { /* … */ })

Example: In this example, The nonRepeatedElement function uses a Map to count element occurrences in an array using map(). It then finds and returns the first non-repeated element from the original array, or null if none.

function nonRepeatedElement(arr) {
    /* Create a map to store 
    the count of each element */
    const elementCount = new Map();

    /* Use map to count the occurrences 
    of each element in the array */
    arr.map((element) =>
        elementCount.has(element)
            ? elementCount.set(
                  element,
                  elementCount.get(element) + 1
              )
            : elementCount.set(element, 1)
    );

    /* Iterate through the array again 
    to find the first non-repeated element */
    for (let i = 0; i < arr.length; i++) {
        if (elementCount.get(arr[i]) === 1) {
            return arr[i];
        }
    }

    /* If no non-repeated element is 
    found, return null or a default value */
    return null;
}

const array = [9, 4, 9, 6, 7, 4];
const result = nonRepeatedElement(array);
console.log(result);

6

Approach 4: Using Array Filter Method:

The approach filters elements by comparing their first and last occurrences in the array. Elements with different first and last indexes are returned, ensuring only non-repeated elements are included.

Syntax:

array.filter(callback(element, index, arr), thisValue)

Example: In this example we finds the first non-repeated element in an array. If found, it prints the element; otherwise, it indicates that all elements are repeated.

let arr = [9, 4, 9, 6, 7, 4];

let nonRepeated = arr.filter((num) =>
    arr.indexOf(num) === arr.lastIndexOf(num)
)[0];

let result = nonRepeated !== undefined
    ? "The first non-repeated element is: " + nonRepeated
    : "All elements in the array are repeated.";

console.log(result);

The first non-repeated element is: 6

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