The map operation on array expects a call back as a parameter (which itself consumes an element of the array)
So, instead of going with the one line notation, lets try this,
const numbers = [ 1, 4, 9]
const sqrt = num => Math.sqrt(num)
const roots = numbers.map(sqrt)
The sqrt function, takes a number takes a num returns the root.
The map function consumes this sqrt function and performs it on all elements.
What you wrote up there, is practically is a shorter version (maybe because we are lazy, maybe because we want to save namespace)