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What’s your alternative solution? Challenge #28

codeguppy profile image Adrian ・2 min read

About this series

This is series of daily JavaScript coding challenges... for both beginners and advanced users.

Each day I’m gone present you a very simple coding challenge, together with the solution. The solution is intentionally written in a didactic way using classic JavaScript syntax in order to be accessible to coders of all levels.

Solutions are designed with increase level of complexity.

Today’s coding challenge

Create a function that will receive two arrays and will return an array with elements that are in the first array  but not in the second

(scroll down for solution)

Code newbies

If you are a code newbie, try to work on the solution on your own. After you finish it, or if you need help, please consult the provided solution.

Advanced developers

Please provide alternative solutions in the comments below.

You can solve it using functional concepts or solve it using a different algorithm... or just solve it using the latest ES innovations.

By providing a new solution you can show code newbies different ways to solve the same problem.


// Solution for challenge26

var ar1 = [1, 2, 3, 10, 5, 3, 14];
var ar2 = [-1, 4, 5, 6, 14];

var ar = mergeLeft(ar1, ar2);

function mergeLeft(ar1, ar2)
    var ar = [];

    for(let el of ar1)
        if (!ar2.includes(el))

    return ar;

To quickly verify this solution, copy the code above in this coding editor and press "Run".

Note: The solution was originally designed for environment, and therefore is making use of println. This is the almost equivalent of console.log in other environments. Please feel free to use your preferred coding playground / environment when implementing your solution.

Discussion (2)

Editor guide
themobiledev profile image
Chris McKay • Edited

So, here is my plain JavaScript solution:

var ar1 = [1, 2, 3, 10, 5, 3, 14];
var ar2 = [-1, 4, 5, 6, 14];

println(mergeLeft(ar1, ar2));

function mergeLeft(ar1, ar2) 
    return ar1.filter(e => !ar2.includes(e));
michi profile image
Michael Z • Edited

To clearly show the intent of such operations I created my own little library flooent.

import { given } from 'flooent'


If it was an array of objects and we are comparing the field key:

import { given } from 'flooent'

given(array1).whereNotIn('key', array2)