Javascript parsing int64

Question

How can I convert a long integer (as a string) to a numerical format in Javascript without javascript rounding it?

var ThisInt = '9223372036854775808'
alert(ThisInt+'\r' +parseFloat(ThisInt).toString()+'\r' +parseInt(ThisInt).toString());

I need to perform an addition on it before casting it back as a string & would prefer not to have to slice it two if at all possible.

Answer 1

All Numbers in Javascript are 64 bit "double" precision IEE754 floating point.

The largest positive whole number that can therefore be accurately represented is 2^53 - 1. The remaining bits are reserved for the exponent.

Your number is exactly 1024 times larger than that, so loses 3 decimal digits of precision. It simply cannot be represented any more accurately.

In ES6 one can use Number.isSafeInteger( # ) to test a number to see if its within the safe range:

var ThisInt = '9223372036854775808'; 
console.log( Number.isSafeInteger( parseInt( ThisInt ) ) );

There is also a BigInteger library available which should be able to help, though, and avoid you having to do all the string and bit twiddling yourself.

EDIT 2018/12 there's now a native BigInt class (and new literal syntax) landed in Chrome and NodeJS.

Answer 2

With a little help from recursion, you can directly increment your decimal string, be it representing a 64 bit number or more...

/**
 * Increment a decimal by 1
 *
 * @param {String} n The decimal string
 * @return The incremented value
 */
function increment(n) {
    var lastChar = parseInt(n.charAt(n.length - 1)),
        firstPart = n.substr(0, n.length - 1);

    return lastChar < 9
        ? firstPart + (lastChar + 1)
        : firstPart
            ? increment(firstPart) + "0"
            : "10";
}

Answer 3

You cannot do this with standard Javascript. But as always, there is a nifty little library to help us out, in this case BigInt.js, which will let you use arbitrary-precision integers.