Answer 1

You need to generate a random number from 0 to array.length and use that as an index.

Math.random() will return a number between 0 and 1, so all you need to do is multiply and round down to a whole number.

let random_index = Math.floor(Math.random() * my_array.length);
let my_random_element = my_array[random_index];

Answer 2

Here's how to use JavaScript's random() function:
Math.floor((Math.random() * maxValue) + minNumber);

For example, for an array with 100 items:
Math.floor((Math.random() * 100); //Gives 0 - 99