## JavaScript Integer math incorrect results

I am just trying to implement a simple RNG in JS.

What's happening is javascript evaluates `119106029 * 1103515245`

to be `131435318772912110`

rather than `131435318772912105`

. We know it's wrong since two odd numbers multiplied does not give an even number.

Anyone know what's up? I just want a reliable repeatable RNG, and because of these incorrect values I can't get results to match up with my C implementation of the same thing.

Per the ECMAScript standard, all numbers in JavaScript are (64-bit IEEE 754) floating-point numbers.

However all 32-bit integers can be exactly represented as floating-point numbers. You can force a result to 32 bits by using the appropriate bitwise operator, like this:

x = (a * b) >>> 0; // force to unsigned int32 x = (a * b) | 0; // force to signed int32

Weird, but that's the standard.

(Incidentally this rounding behavior is one of the most frequently reported "bugs" against Firefox's JavaScript engine. Looks like it's been reported 3 times so far this year...)

As for reproducible random numbers in JavaScript, the V8 benchmark uses this:

// To make the benchmark results predictable, we replace Math.random // with a 100% deterministic alternative. Math.random = (function() { var seed = 49734321; return function() { // Robert Jenkins' 32 bit integer hash function. seed = ((seed + 0x7ed55d16) + (seed << 12)) & 0xffffffff; seed = ((seed ^ 0xc761c23c) ^ (seed >>> 19)) & 0xffffffff; seed = ((seed + 0x165667b1) + (seed << 5)) & 0xffffffff; seed = ((seed + 0xd3a2646c) ^ (seed << 9)) & 0xffffffff; seed = ((seed + 0xfd7046c5) + (seed << 3)) & 0xffffffff; seed = ((seed ^ 0xb55a4f09) ^ (seed >>> 16)) & 0xffffffff; return (seed & 0xfffffff) / 0x10000000; }; })();

**Avoiding Problems with Decimal Math in JavaScript,** But even though we get the correct result, using toFixed means that we are just Division of integers and multiplication by decimals may still result in inexact Definition and Usage. The floor() method rounds a number DOWNWARDS to the nearest integer, and returns the result. If the passed argument is an integer, the value will not be rounded.

When an integer in javascript is too big to fit in a 32 bit value, some browsers will convert it to a floating point. Since the value of floating points is only save to a limited precision, some rounding can occur on big values.

**JS WTF 🦄 with Math,** Let's ride on weirdness with JavaScript Math. In the case of integer values, only integers that can be represented as By definition, ToNumber returns 1 if the argument is true, 0 if the argument is false. The final result is 2 . Avoiding Problems with Decimal Math in JavaScript. Originally published in the A Drip of JavaScript newsletter. One of the more unintuitive aspects of JavaScript, particularly for new developers, is the fact that decimal math doesn't always work as you'd expect it to.

If done in C/C++ (double), the last numbers will be ...112 instead of 105 (which is correct). If performed with 'long double', the result will be as expected (...105). So it looks like the Javascript interpreter converts the numbers to 8-byte-double internally, does the calculation and does some unknown rounding which leads to a marginally better result than the C/C++ standard double calculation.

GCC 4.5:

int main(int argc, char** argv) { long double a = 119106029; long double b = 1103515245; long double c = a * b; printf("%.Lf\n", c); return 0; }

Result:

131435318772912105

Expected:

131435318772912105

So I don't see a chance in Javascript without the aid of a BIGNUM library (if any).

Regards

rbo

**Number.isInteger(),** If the target value is an integer, return true , otherwise return false . return typeof value === 'number' && isFinite(value) && Math.floor(value) JavaScript has many familiar operators from basic math, as well as a few additional operators specific to programming. Here is a reference table of JavaScript arithmetic operators. Operator

With the arrival of BigInt, you can now perform these calculations with accuracy:

console.log((119106029n * 1103515245n).toString());

**parseFloat(),** JavaScript Demo: Standard built-in objects - parseFloat(). xxxxxxxxxx. 1. function circumference(r) {. 2. return parseFloat(r) * 2.0 * Math.PI; same as if parseFloat had been called on the result of those methods. The following examples both return 900719925474099300 , losing precision as the integer is The operands of all bitwise operators are converted to signed 32-bit integers in two's complement format, except for zero-fill right shift which results in an unsigned 32-bit integer. Two's complement format means that a number's negative counterpart (e.g. 5 vs. -5 ) is all the number's bits inverted (bitwise NOT of the number, or ones

**Chapter 11. Numbers,** A number literal can be an integer, floating point, or (integer) hexadecimal: How can we make sure that results of arithmetic computations are correct? Math.E // returns Euler's number Math.PI // returns PI Math.SQRT2 // returns the square root of 2 Math.SQRT1_2 // returns the square root of 1/2 Math.LN2 // returns the natural logarithm of 2 Math.LN10 // returns the natural logarithm of 10 Math.LOG2E // returns base 2 logarithm of E

**Equality,** The largest integer number which can be represented by a JavaScript In most cases, round-off errors don't matter: they have no significant impact on the results. For example executing 0.1 + 0.2 == 0.3 in JavaScript will return false, as the Math.atan2 Math.ceil Math.pow Math.round In some cases,it's possible for a -0 to be introduced into an expression as a return value of these methods even when no -0 exists as one of the parameters. For example, using Math.pow to raise -Infinity to the power of any negative, odd exponent evaluates to -0. Refer to the documentation for the

**11. Numbers - Speaking JavaScript [Book],** A number literal can be an integer, floating point, or (integer) hexadecimal: How can we make sure that results of arithmetic computations are correct? Using it without checking its existence, such as Math.max(Number.MAX_SAFE_INTEGER, 2), will yield undesired results such as NaN. Because MAX_SAFE_INTEGER is a static property of Number , you always use it as Number.MAX_SAFE_INTEGER , rather than as a property of a Number object you created.