Got it, thanks!!

1. The filter method for arrays receives up to 3 arguments (in the following order) element, index, array and returns a filtered array in which the function passed in returns true
2. The arrow => denotes an arrow function, introduced in ES6, the newest specification for Javascript to date. The behaviour and usage of arrow functions is nearly identical to that of ordinary functions - the notations can be used interchangeably 99% of the time
3. The three dots prefixed to a variable ... adds all the items in a specified list (e.g. function arguments, an array or even a string) into an array.

Hope this helps :)

Could you explain how this line works?

copy = [...copy.filter((a,b)=>b>0), ...copy.filter((a,b)=>b==0)];

Thanks!