Why JavaScript base-36 conversion appears to be ambiguous

Question

I am currently writing a piece of JavaScript that uses base 36 encoding.

I came across this problem:

parseInt("welcomeback",36).toString(36)

Appears to return "welcomebacg".

I tested this in the Chrome developer's console and Node.js with the same result.

Is there any logical explanation for this result?

Answer 1

The result of parseInt("welcomeback",36) is larger than Number.MAX_SAFE_INTEGER (2⁵³-1) and thus cannot be accurately represented. A possible workaround is to perform base conversion with BigInt manually.

const str = "welcomeback";
const base = 36;
const res = [...str].reduce((acc,curr)=>
   BigInt(parseInt(curr, base)) + BigInt(base) * acc, 0n);
console.log(res.toString());
console.log(res.toString(36));