Refrain from using global variables

Anyways 'Looks like you've solved it'

"A higher order function is any function that takes a function as a parameter or returns a function as its result." These HOCs allow us to abstract some operations that are being performed repeatedly (in this kata, we want to multiply each index's value by two). Instead of, for example, writing a for loop, we can use map, a classic example of an HOC. map takes an array and a function as arguments, which then uses this function to transform each item in the array, returning a new array with the transformed data. We like that it returns a new array because we want to avoid mutating any data. HOC's can do more than just manipulate array, for example, Function.prototype.bind is another HOC in JavaScript. Just keep practicing, find really good examples of what it is that you don't understand, and try to find the meaning in them and why they're useful. These things don't come naturally for everyone, and it took me a while to understand and appreciate some concepts of programming until I had real life use cases for them.
Sources:

• https://medium.com/javascript-scene/higher-order-functions-composing-software-5365cf2cbe99
• https://hackernoon.com/effective-functional-javascript-first-class-and-higher-order-functions-713fde8df50a
• https://eloquentjavascript.net/05_higher_order.html

That's no kata bug, your code doesn't work, why don't you read the instructions again? I've even added a test to the sample tests.

A digital root is the recursive sum of all the digits in a number. Given n, take the sum of the digits of n. If that value has two digits, continue reducing in this way until a single-digit number is produced.

Depending on the input, your code returns a number with more than one digit.

Make sure to understand where JavaScript came from before you dive into some of the newer stuff :)