Reduce and filter refusing to co-operate!

As a newbie, I’m trying to use iterative methods like reduce and filter, in order to get familiar with their operation.

Using both with Sorted Union (Intermediate Algorithms), I can’t spot why my code is failing to work.

I’m guessing some simple-level mistake here, but can’t see it. So could use a bit of help from more experienced coders.

My code:

function uniteUnique(arr) {
 let newArr = [...(arr)]  //create new, flattened arr
     .reduce(function(accum, currVal) {
     return accum.concat(currVal)
   }, [] );
var resultArr = newArr //filter out repeated values  
   .filter(function(item, pos) {
     return newArr.indexOf(item) == pos;
   return resultArr;

uniteUnique([1, 3, 2], [5, 2, 1, 4], [2, 1]);

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Challenge: Sorted Union

Link to the challenge:

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Take a look at what your functions are doing - it looks like they only access the first sub-array:


Thanks, I’ll get onto it now.

Yes, you’re right.

Back to the drawing board…

Many thanks,


Changed first line to

let newArr = Array.from(arguments) //the rest I left intact

Worked beautifully!

I had tried a similar first line earlier, but for some reason it didn’t produce the goods.

I find this new field of study to be fiendishly awkward, especially because, when checking code, one can not be sure which elements of the code are going wrong, from a simple syntax error, to a more abstruse misunderstanding of (nested) functions/methods, and the like.

As a linguist (in an earlier life) , it’s a bit like learning 3 languages at once, then combining them.

Your help is much appreciated!


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