BigInt-compatible Base62 Conversion Functions

base62.js

// requires ES2020 for BigInt support

const digits = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

function toBase62(number) {
  if (number === 0n) {
    return '0';
  }
  let result = '';
  let remaining = number;
  while (remaining > 0n) {
    result = digits[Number(remaining % 62n)] + result;
    remaining = remaining / 62n;
  }
  return result;
}

function fromBase62(string) {
  const length = BigInt(string.length);
  return [...string].reduce((accumulator, character, index) => {
    const decodedCharacter = BigInt(digits.indexOf(character));
    if (decodedCharacter < 0) {
      throw new Error('Input string contained non-base62 characters');
    }
    return (
      accumulator + decodedCharacter * 62n ** (length - BigInt(index) - 1n)
    );
  }, 0n);
}

base62.ts

const digits = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

function toBase62(number: bigint): string {
  if (number === 0n) {
    return '0';
  }
  let result = '';
  let remaining = number;
  while (remaining > 0n) {
    result = digits[Number(remaining % 62n)] + result;
    remaining = remaining / 62n;
  }
  return result;
}

function fromBase62(string: string): bigint {
  const length = BigInt(string.length);
  return [...string].reduce(
    (accumulator: bigint, character: string, index: number): bigint => {
      const decodedCharacter = BigInt(digits.indexOf(character));
      if (decodedCharacter < 0) {
        throw new Error('Input string contained non-base62 characters');
      }
      return (
        accumulator + decodedCharacter * 62n ** (length - BigInt(index) - 1n)
      );
    },
    0n,
  );
}