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.

Determined

Number.MAX_SAFE_INTEGER ... Number.MIN_SAFE_INTEGER – Toot

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.

EDIT 2023/11 the BigInt class has been supported in all mainstream JS engines since late 2020.

Hom

Thank you. He has a lot of useful functions which I can see are going to help me for some time to come. Cheers – Determined

you're welcome! The fact that all numbers in JS are actually doubles is fundamental to the language, but apparently not well known. – Hom

@Hom If the largest positive whole number is 2^53, how is it that parseInt() can return bigger than that? – Consuetudinary

@Consuetudinary for larger numbers you'll get an approximation, with missing bits in the least significant positions. – Hom

In Firefox Quantum 58.0.2, running javascript:(alert(Math.pow(2, 53))) returns 9007199254740992, and running javascript:(alert(-Math.pow(2, 53))) returns -9007199254740992. If I add 1 to the max or subtract 1 from the min, it returns those same numbers. – Toot

From functions support perspective, bn.js much more than BigInteger - actually many packages used in browserify, such as crypto are using it. – Singleminded

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";
}

Chrisom

@x3ro: Why? It's all in getting used to proper indentation. Terse syntax that's following a convention is often better than verbose code that takes ages to mentally parse. I think you are blindly applying conventional wisdom (that "terse is bad") without considering the usage... – Chrisom

@Ates Goral: No, not really (to the applying conventional wisdom). I see why it is done like this, however I think that it decreases the readability of your code significantly, and thats why I dislike it. But maybe I'm a bit damaged too, because I had to debug code with like 10 nested ternary operators, and it wasn't that much fun. – Winy

@Ates Goral: Yeah, true. However, imagine you need a quick change in such a piece of code. Probably you would rather add another ternary than rewrite it, wouldn't you? :) – Winy

+1 for a functional suggestion. I don't like needless complexity in general, and this could be argued as being that, but that doesn't mean it's a bad solution. It just means that if I don't want to allow it, I'll say so in our coding standards. That is, afterall, what they're for. – Dominickdominie

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.

Winy

Answer 4

Have you tried using the Number class?
var num = new Number(parseFloat(ThisInt))

Lette

Answer 5

Just use Number(ThisInt) for this instead of Int or float

Cocteau

-1 Does not work in Chrome: number("9223372036854775808") and NUMBER("9223372036854775808") both fail. – Winy

@x3ro - Check this article, It says Chrome is also supported - w3schools.com/jsref/jsref_Number.asp – Cocteau

@Sachin Shanbhag: Yep, but the way you answered one would suspect Number to be a function rather than an object of which you need to create an instance with new – Winy

@x3ro - Hey, no I did not mean to create instance of object tough. Check the article, Number() function can be used directly without having to create instance. This is indeed a global function. – Cocteau

@Sachin Shanbhag: Ah okay, my bad. Anyhow, new Number() and Number() seem to be identical. – Winy