If I understand the comments well here's the answer

 function calcTime (digits, num ){
        const digits_arr = Array.from(digits);
        let last_index = 0, new_index, time =0;
        for (const n of num) {
            new_index =digits_arr.findIndex(x => x===n);
            time += Math.abs(new_index - last_index);
            last_index = new_index;
        }
        return time;
    }

example: Input: digits = "0123456789", num = "201" Output: 4

Dude, I suppose you didn't have much time to explain what you where trying to solve. But anyway I run into this post because I was trying to understand how to start to solve the problem that I've found and thanks to @AbdelazizAlsabagh I've finally understand the trick with the indexes!. I'm posting the full problem to solve (but I haven't try the solution yet, I'll updated later):

Number Generator

A digit-only keyboard contains all 10 digits from 0 to 9. They all exist in one line. Give a string of 10 digits illustrating how the keys are positioned. To type a digit, you start from index zero to the index of the target digit. It takes |a - b| milliseconds to move from index a to index b.

Write a function to calculate the number of milliseconds needed to type a number with one finger.

Input: digits = '0123456789', num = '210

Output: 4

Input: digits = '8459761203', num = '5439'

Output: 17

Constraints:

• digits.length == 10
• digits contains each digit [0-9] exactly one in some order
• 1 <= num.length <= 10e4
• num[i] is digit

def number_gen_idx(digits: str, nums: str) -> str:
    """
    >>> number_gen_idx('0123456789', '210')
    4
    >>> number_gen_idx('8459761203', '5439')
    17
    """
    digits_idx = defaultdict(int)
    for idx, digit in enumerate(digits):
        digits_idx[digit] = idx
    curr_ptr = 0
    result = 0
    for digit in nums:
        result += abs((digits_idx[digit] - curr_ptr))
        curr_ptr = digits_idx[digit]
    return result