Picking out uniques from an array in JavaScript

Suppose we have an array that contains duplicate elements like this ?

const arr = [1,1,2,2,3,4,4,5];

We are required to write a JavaScript function that takes in one such array and returns a new array. The array should only contain the elements that only appear once in the original array.

Therefore, let's write the code for this function ?

Method 1: Using indexOf() and lastIndexOf()

This approach compares the first and last occurrence of each element. If they're different, the element appears multiple times:

const arr = [1,1,2,2,3,4,4,5];
const extractUnique = arr => {
    const res = [];
    for(let i = 0; i < arr.length; i++){
        if(arr.lastIndexOf(arr[i]) !== arr.indexOf(arr[i])){
            continue;
        };
        res.push(arr[i]);
    };
    return res;
};
console.log(extractUnique(arr));

[ 3, 5 ]

Method 2: Using filter() with indexOf() and lastIndexOf()

A more concise approach using the filter method:

const arr = [1,1,2,2,3,4,4,5];
const extractUnique = arr => {
    return arr.filter((item, index) => 
        arr.indexOf(item) === arr.lastIndexOf(item)
    );
};
console.log(extractUnique(arr));

[ 3, 5 ]

Method 3: Using Map to Count Occurrences

This method counts each element's occurrences and filters those appearing exactly once:

const arr = [1,1,2,2,3,4,4,5];
const extractUnique = arr => {
    const count = new Map();
    
    // Count occurrences
    arr.forEach(item => {
        count.set(item, (count.get(item) || 0) + 1);
    });
    
    // Filter items that appear only once
    return arr.filter(item => count.get(item) === 1);
};
console.log(extractUnique(arr));

[ 3, 5 ]

Comparison

Conclusion

For small arrays, use the filter method for cleaner code. For larger datasets, the Map-based approach offers better performance with O(n) complexity.