How to round numbers to million, thousand, hundred, 1, 0.1, 0.01 and so on? possibly in JS?

Question

is there any mathematical formula for this kind of rounding? example:

1,234,567 → 1,000,000
9,321,232 → 1,000,000

12,230 → 10,000
90,000 → 10,000

1,234 → 1,000
9,000 → 1,000

9 → 1

0.12321 → 0.1
0.9123 → 0.1

0.01321 → 0.01
0.09321 → 0.01

and so on

Thank you

Answer 1

Just get the most significant digit with log base 10 floored, then raise 10 to that power.

const nums = [
  1_234_567,
  9_321_232,
  12_230,
  90_000,
  1_234,
  9_000,
  9,
  0.12321,
  0.9123,
  0.01321,
  0.09321,
];

const weirdRound = (n) => 10 ** (Math.floor(Math.log10(n)));

console.log(nums.map(weirdRound));