GH-45250: [JS] Fix denominator precision loss and remove unnecessary …

…safe integer check for fractional part (#45251)

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### Rationale for this change

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### What changes are included in this PR?

This PR uses string constructor for bigint to calculate the power of 10
with scale as the exponent value of the expression.

To Fix precision loss like

```typecript
> BigInt(10) ** BigInt(25);
10000000000000000000000000n
> BigInt(Math.pow(10, 25))
10000000000000000905969664n
```

Also, we remove the unnecessary safe integer check for the fraction
part.



### Are these changes tested?

add some unit tests

### Are there any user-facing changes?

no
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<!-- **This PR contains a "Critical Fix".** -->
* GitHub Issue: #45250

---------

Co-authored-by: Paul Taylor <[email protected]>

js/src/util/bn.ts

@@ -91,11 +91,14 @@ export function bigNumToNumber<T extends BN<BigNumArray>>(bn: T, scale?: number)
             number |= word * (BigInt(1) << BigInt(64 * i++));
         }
     }
-    if (typeof scale === 'number') {
-        const denominator = BigInt(Math.pow(10, scale));
+    if (typeof scale === 'number' && scale > 0) {
+        const denominator = BigInt('1'.padEnd(scale + 1, '0'));
         const quotient = number / denominator;
-        const remainder = number % denominator;
-        return bigIntToNumber(quotient) + (bigIntToNumber(remainder) / bigIntToNumber(denominator));
+        const remainder = negative? -(number % denominator) : number % denominator;
+        const integerPart = bigIntToNumber(quotient);
+        const fractionPart = `${remainder}`.padStart(scale, '0');
+        const sign = negative && integerPart === 0 ? '-' : '';
+        return +`${sign}${integerPart}.${fractionPart}`;
     }
     return bigIntToNumber(number);
 }

js/test/unit/bn-tests.ts

@@ -16,7 +16,7 @@
 // under the License.
 
 import * as Arrow from 'apache-arrow';
-const { BN } = Arrow.util;
+const { BN, bigNumToNumber } = Arrow.util;
 
 describe(`BN`, () => {
     test(`to detect signed numbers, unsigned numbers and decimals`, () => {
@@ -98,4 +98,15 @@ describe(`BN`, () => {
         // const n6 = new BN(new Uint32Array([0x00000000, 0x00000000, 0x00000000, 0x80000000]), false);
         // expect(n6.valueOf(1)).toBe(1.7014118346046923e+37);
     });
+
+    test(`bigNumToNumber`, () => {
+        const n1 = new BN(new Uint32Array([3, 2, 1, 0]));
+        expect(() => bigNumToNumber(n1)).toThrow('18446744082299486211');
+        /* eslint-disable @typescript-eslint/no-loss-of-precision */
+        expect(bigNumToNumber(n1, 10)).toBeCloseTo(1844674408.2299486);
+        expect(bigNumToNumber(n1, 15)).toBeCloseTo(18446.744082299486);
+        expect(bigNumToNumber(n1, 20)).toBeCloseTo(0.18446744082299486);
+        expect(bigNumToNumber(n1, 25)).toBeCloseTo(0.0000018446744082299486);
+        /* eslint-enable @typescript-eslint/no-loss-of-precision */
+    });
 });