No. The requirement is to 'filter out the strings'. We don't really care if the left overs are numbers, objects, or anything else for that matter. Just don't include the strings!

Found the answer.

it's not the spread operator ..., but looks like it. using ... in the function parameter definition makes it a 'rest parameter' that creates an array of all arguments AND creates that arguments object as usual.

Useful for variadic functions with a unknown number of parameters.

Reference: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/rest_parameters

Can you help me understand the spread operator and reduce here.

a is the set of arguments, and ... will 'spread' the set of arguments as parameters to the function.

However, the how is a.reduce possible? a will be the arguments object as far as I understand, and arguments is 'arrray-like', but not an array, and will usually throw an error that 'reduce' is not a function on arguments.

What's happening here?

I see, then these websites are wrong, when they state that the let keyword is part of the ES6 and newer:

https://medium.freecodecamp.com/5-javascript-bad-parts-that-are-fixed-in-es6-c7c45d44fd81#.xuzvyk491
https://strongloop.com/strongblog/es6-variable-declarations/
https://davidwalsh.name/for-and-against-let

Sorry, just a little confused.

I used recursion, but I assumed that you'd need to have a check on whethere "next" is null, but of course null is falsey, so the concat should just fail at the end. Nice, succinct.

This will only work for ES6, right?

Additionally, I didn't even think about putting a conditional test as the property value in an object! Very succinct and novel.