kr_numerals.ts

const magnitudes = [
  {magnitude: 1_0000_0000, suffix: "억"},
  {magnitude: 1_0000, suffix: "만"},
  // {magnitude: 1000, suffix: "천"}, // this works too
];

const krFormat = (num: number): string => {
  const m = magnitudes.find((m) => num >= m.magnitude);
  if (!m) {
    return `${num}`
  }
  const v = num / m.magnitude;
  return `${v}${m.suffix}`
};

[
  1,
  10,
  100,
  1_000,
  10_000,
  100_000,
  1_000_000,
  10_000_000,
  100_000_000,
  1_000_000_000,
].forEach((v) => {
  console.log(`${v} = ${krFormat(v)}`)
})

Run

$ deno run kr_numerals.ts

Output

1 = 1
10 = 10
100 = 100
1000 = 1000
10000 = 1만
100000 = 10만
1000000 = 100만
10000000 = 1000만
100000000 = 1억
1000000000 = 10억

...or with the {magnitude: 1000, suffix: "천"} commented out (seems less usual):

1 = 1
10 = 10
100 = 100
1000 = 1천
10000 = 1만
100000 = 10만
1000000 = 100만
10000000 = 1000만
100000000 = 1억
1000000000 = 10억

👍🙇‍♂️😍❤️