these are the same thing as above, except now with arrow notation
~~~1~~~
[-1, 2, 4, -9].filter(val => val < 0)
~~~2~~~
[-1, 2, 4, -9].filter(x => { return x < 0; })
~~~3~~~
const filterFunction = (num) => num < 0;
var filtered = [-1, 2, 4, -9].filter(filterFunction)