I have the following code:

```
var str = "0x4000000000000000"; //4611686018427387904 decimal
var val = parseInt(str);
alert(val);
```

I get this value: "`4611686018427388000`

", which is `0x4000000000000060`

I was wondering if JavaScript is mishandling 64-bit integers or am I doing something wrong?

JavaScript represents numbers using IEEE-754 double-precision (64 bit) format. As I understand it this gives you 53 bits precision, or fifteen to sixteen decimal digits. Your number has more digits than JavaScript can cope with, so you end up with an approximation.

This isn't really "mishandling" as such, but obviously it isn't very helpful if you need full precision on large numbers. There are a few JS libraries around that can handle larger numbers, e.g., BigNumber and Int64.

Chromium version 57 and later natively supports arbitrary-precision integers. This is called BigInt and is being worked on for other browsers as well. It is dramatically faster than JavaScript implementations.

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