为什么 JavaScript base-36 转换看起来不明确

我目前正在编写一段使用 Base 36 编码的 JavaScript。

我遇到了这个问题：

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

看来要回归了"welcomebacg".

我在 Chrome 开发者控制台和 Node.js 中对此进行了测试，结果相同。

这个结果有什么合乎逻辑的解释吗？

The result of parseInt("welcomeback",36) is larger than Number.MAX_SAFE_INTEGER https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/MAX_SAFE_INTEGER (2⁵³-1) and thus cannot be accurately represented. A possible workaround is to perform base conversion with BigInt https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/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));