Base 36 to BigInt?

Question

Suppose I want to convert a base-36 encoded string to a BigInt, I can do this:

BigInt(parseInt(x,36))

But what if my string exceeds what can safely fit in a Number? e.g.

parseInt('zzzzzzzzzzzzz',36)

Then I start losing precision.

Are there any methods for parsing directly into a BigInt?

Answer 1

You could convert the number to a bigint type.

function convert(value, radix) {
    return [...value.toString()]
        .reduce((r, v) => r * BigInt(radix) + BigInt(parseInt(v, radix)), 0n);
}

console.log(convert('zzzzzzzzzzzzz', 36).toString());

With greater chunks, like just for example with ten (eleven return a false result).

function convert(value, radix) { // value: string
    var size = 10,
        factor = BigInt(radix ** size),
        i = value.length % size || size,
        parts = [value.slice(0, i)];

    while (i < value.length) parts.push(value.slice(i, i += size));

    return parts.reduce((r, v) => r * factor + BigInt(parseInt(v, radix)), 0n);
}

console.log(convert('zzzzzzzzzzzzz', 36).toString());

Answer 2

Not sure if there's a built-in one, but base-X to BigInt is pretty easy to implement:

function parseBigInt(
  numberString,
  keyspace = "0123456789abcdefghijklmnopqrstuvwxyz",
) {
  let result = 0n;
  const keyspaceLength = BigInt(keyspace.length);
  for (let i = 0; i < numberString.length; i++) {
    const value = keyspace.indexOf(numberString[i]);
    if (value === -1) throw new Error("invalid string");
    result = result * keyspaceLength + BigInt(value);
  }
  return result;
}

console.log(parseInt("zzzzzzz", 36));
console.log(parseBigInt("zzzzzzz"));
console.log(parseBigInt("zzzzzzzzzzzzzzzzzzzzzzzzzz"));

outputs

78364164095
78364164095n
29098125988731506183153025616435306561535n

The default keyspace there is equivalent to what parseInt with base 36 uses, but should you need something else, the option's there. :)