How to create a new array based on the number of sub arrays in js

Here is my first array:

const withSub = [[31, 32, 34, 34], [32, 36], [12], [15, 38]];

Here is my second array:

const arr = ['f', 'b', 'c', 'd'];

The number of arrays in withSub is always equal to the number of arr arrays, while the number of subarrays in withSub may be different.

I want to create a function that generates a new array based on these two arrays in such a way that the number of each element in arr must be equal to the number of subarrays in withArr.

In this example the final array must be:

   ['f', 'f', 'f', 'f', 'b', 'b', 'c', 'd', 'd'];