functionrandomPrefix(x,n,fr)// x:  an array// n:  number of values [-1 ⇒ any]// fr: random number generator ([0, 1))

constxarray=require('extra-array');varx=[1,2,3];xarray.randomPrefix(x);// → [ 1, 2 ]xarray.randomPrefix(x,1);// → [ 1 ]xarray.randomPrefix(x,-1);// → [ 1, 2 ]