.pop() is not a function

Someone commented on my crib sheet to say (quote edited to match the numbers here):

This is element-wise multiplication, so each element in the array is multiplied by 5. If the goal is to repeat the array, so you have [0.4, -0.4, 0.4, -0.4…] then use:

Array(5).fill([0.4,-0.4]).flat()

Comments?