Feature request: parseInt for bigint (convert any string of any radix)

💡 Ideas
1

I was thinking of adding a function that is similar to parseint(string, radix) but works with bigint. So far according to mozilla, bigint() only works with binary (0b), decimal, and hex(0x). Something like parseBigInt(string, radix).

2

Interesting feature, and curious what's the specific use case for this function? A string stored a number that's larger than 2^53-1 seems taking much memory, except for the radix are really possible to be large enough, too.

Otherwise, an alternative for converting a valid integer string to BigInt type could be:

BigInt(parseInt('12345', radix)) // 12345n
3

That has a potential of losing precision if the string is a large number. I am suggesting a function that converts a string with any radix to bigint without losing precision.

You could try to make it so up to radix=16 is allowed to convert a string to bigint.

4

See https://github.com/tc39/proposal-number-fromstring

5

Thanks!

6

Sorry to rehash an old post, but I also have the use case to support base-36 decoding into a BigInt. I see the link above for fromString hasn't moved in a while. Does anyone know an unofficial implementation of a parseBigInt function?

7

I figured out an implementation:

const alphabet = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
const base = 36;
const chars = Array.from(s).reverse();
return chars.reduce((c, n, i) => {
  const pos = BigInt(alphabet.indexOf(n));
  return c + (pos * BigInt(Math.pow(base, i)));
}, 0n);
1 Like
8

Math.pow(base, i) will lose precision quickly. Math.pow(36, 17) != 36n ** 17n. Good news is you don't need to use exponentiation at all if you don't reverse the input.

function parse(str, base = 36n) {
  return Array.prototype.reduce.call(str, (acc, digit) => {
    const pos = BigInt(alphabet.indexOf(digit));
    return acc * base + pos;
  }, 0n);
}
3 Likes
9

Nice, much cleaner. I overlooked the Math.pow issue. Thanks!

10

And if you did need to reverse the input, you could just use .reduceRight.