idx is usually short for index.

Python loops allows items in a nested list to be accessed directly like so:

>>> lst = [[1, 2], [3, 4], [5, 6]]

>>> 

>>> for a,b in lst:

        print a,b



1 2

3 4

5 6

Using enumerate in Python allows for something similar:

>>> for idx,val in enumerate(['a','b','c']):

        print('index of ' + val + ': ' + str(idx))



index of a: 0

index of b: 1

index of c: 2

The equivalent of enumerate(array) in JavaScript is array.entries(), and can be used in much the same way as Python:

zone_indices = []



for (let i = 0; i < maxz.length, i++) {

    for (let [idx, val] of classified.entries()) {

        if (zone+1 === val) {

            zone_indices.push(idx);

        };

    };

};