GH-45250: [JS] Fix denominator precision loss and remove unnecessary safe integer check for fractional part

js/src/util/bn.ts

@@ -91,15 +91,25 @@ 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' + '0'.repeat(scale));
         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 = padStart((remainder).toString(), scale);
+        const sign: string = negative && integerPart === 0 ? '-' : '';
+        return Number(`${sign}${integerPart}.${fractionPart}`);
     }
     return bigIntToNumber(number);
 }
 
+function padStart(str: string, targetLength: number) {
+    while (str.length < targetLength) {
+        str = '0' + str;
+    }
+    return str;
+}
+
 /** @ignore */
 export function bigNumToString<T extends BN<BigNumArray>>(a: T): string {
     // use BigInt native implementation

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 */
+    });
 });