https://leetcode.com/problems/integer-to-english-words/

integer-to-english-words.js

const WORDS_MAP = Object.freeze({
  0: 'zero',
  1: 'one',
  2: 'two',
  3: 'three',
  4: 'four',
  5: 'five',
  6: 'six',
  7: 'seven',
  8: 'eight',
  9: 'nine',
  10: 'ten',
  11: 'eleven',
  12: 'twelve',
  13: 'thirteen',
  14: 'fourteen',
  16: 'sixteen',
  17: 'seventeen',
  18: 'eighteen',
  19: 'nineteen',
  20: 'twenty',
  30: 'thirty',
  40: 'forty',
  50: 'fifty',
  60: 'sixty',
  70: 'seventy',
  80: 'eighty',
  90: 'ninety',
});

const LARGE_MAP = Object.freeze([
  [1e9, 'billion'],
  [1e6, 'million'],
  [1e3, 'thousand']
]);

function asWords(n) {
  const isInteger = typeof n === 'number';
  if (!isInteger) {
    return null;
  }
  let value = n;
  const str = [];

  if (value === 0) {
    return WORDS_MAP[value];
  }

  LARGE_MAP.forEach(([key, suffix]) => {
    if (value >= key) {
      str.push(getPrefix(value / key), suffix);
      value %= key;
    }
  });

  if (value > 0) {
    str.push(getPrefix(value));
  }
  return str.join(' ').trim();
}

function getPrefix(n) {
  const str = [];
  let truncated = Math.trunc(n);

  if (truncated >= 100) {
    const key = Math.trunc(truncated / 100);
    str.push(WORDS_MAP[key], 'hundred');
    truncated %= 100;
  }

  if (truncated > 0) {
    if (truncated <= 20) {
      str.push(WORDS_MAP[truncated]);
    } else {
      const key = Math.trunc(truncated / 10) * 10;
      str.push(WORDS_MAP[key]);
      const ones = truncated % 10;
      if (ones > 0) {
        str.push(WORDS_MAP[ones]);
      }
    }
  }
  return str.join(' ').trim();
}

function test(input, expected) {
  const actual = asWords(input);
  if (actual === expected) {
    console.log(`[  ok]: ${actual} === ${expected}`);
  } else {
    console.log(`[fail]: ${actual} !== ${expected}`);
  }
}

test(null, null);
test(0, 'zero');
test(1, 'one');
test(2, 'two');
test(9, 'nine');
test(10, 'ten');
test(20, 'twenty');
test(120, 'one hundred twenty');
test(999, 'nine hundred ninety nine');
test(899, 'eight hundred ninety nine');
test(1e3, 'one thousand');
test(1e4, 'ten thousand');
test(100000, 'one hundred thousand');
test(1e6, 'one million');
test(1e7, 'ten million');
test(123456789, 'one hundred twenty three million four hundred fifty six thousand seven hundred eighty nine');
test(123456000, 'one hundred twenty three million four hundred fifty six thousand');