如何将JavaScript中的数字划分为任意精度(例如小数点28位)

Question

我知道https://floating-point-gui.de/，也知道有许多库可以帮助处理大数，但令人惊讶的是，在除法运算的结果中，我没有找到任何处理小数点以上19位的东西。

我花了几个小时尝试一些库，如精确数学、decimal.js、bignumber.js和其他库。

您将如何处理下面用⭐标记的情况？

// https://jestjs.io/docs/getting-started#using-typescript

import exactMath from 'exact-math'; // https://www.npmjs.com/package/exact-math
import { Decimal } from 'decimal.js'; // https://github.com/MikeMcl/decimal.js

const testCases = [
  // The following cases work in exact-math but fail in Decimal.js:
  '9999513263875304671192000009',
  '4513263875304671192000009',
  '530467119.530467119',
  // The following cases fail in both Decimal.js and exact-math:
  '1.1998679030467029262556391239', // ⭐ exact-math rounds these 28 decimal places to 17: "1.1998679030467029263000000000"
];

describe('decimals.js', () => {
  testCases.forEach((testCase) => {
    test(testCase, () => {
      expect(new Decimal(testCase).div(new Decimal(1)).toFixed(28)).toBe(testCase); // Dividing by 1 (very simple!)
    });
  });
});

describe('exact-math', () => {
  testCases.forEach((testCase) => {
    test(testCase, () => {
      expect(exactMath.div(testCase, 1, { returnString: true })).toBe(testCase); // Dividing by 1 (very simple!)
    });
  });
});

Answer 1

上面的一些测试用例没有意义(因为我不应该期望输出与输入相等，因为我使用的是.toFixed()。

然后，真正的答案是@James:使用建议选项：https://www.npmjs.com/package/exact-math#the-config-maxdecimal-property-usage或decimal.js中的https://mikemcl.github.io/decimal.js/#precision。

请参见下面使用⭐的行。

import exactMath from 'exact-math'; // https://www.npmjs.com/package/exact-math
import { Decimal } from 'decimal.js'; // https://github.com/MikeMcl/decimal.js

/**
 *
 * @param amount {string}
 * @param decimals {number} e.g. 6 would return 6 decimal places like 0.000000
 * @param locale {string} e.g. 'en-US' or 'de-DE'
 * @returns {string} e.g. 1,000.000000
 */
export function getLocaleStringToDecimals(amount: string, decimals: any, locale?: string): string {
  // Thanks to https://stackoverflow.com/a/68906367/ because https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toLocaleString and https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt/toLocaleString would not work for huge numbers or numbers with many decimal places.

  const decimalFormat = new Intl.NumberFormat(locale, { minimumFractionDigits: 1, maximumFractionDigits: 1 });
  const decimalFullString = '1.1';
  const decimalFullNumber = Number.parseFloat(decimalFullString);
  const decimalChar = decimalFormat.format(decimalFullNumber).charAt(1); // e.g. '.' or ','
  const fixed = new Decimal(amount).toFixed(decimals);
  const [mainString, decimalString] = fixed.split('.'); // ['321321321321321321', '357' | '998']
  const mainFormat = new Intl.NumberFormat(locale, { minimumFractionDigits: 0 });
  let mainBigInt = BigInt(mainString); // 321321321321321321n
  const mainFinal = mainFormat.format(mainBigInt); // '321.321.321.321.321.321' | '321.321.321.321.321.322'
  const decimalFinal = typeof decimalString !== 'undefined' ? `${decimalChar}${decimalString}` : ''; // '.357' | '.998'
  const amountFinal = `${mainFinal}${decimalFinal}`; // '321.321.321.321.321.321,36' | '321.321.321.321.321.322,00'
  // console.log({
  //   amount,
  //   fixed,
  //   mainString,
  //   decimalString,
  //   'decimalString.length': decimalString ? decimalString.length : undefined,
  //   decimalFormat,
  //   decimalFinal,
  //   mainFormat,
  //   mainBigInt,
  //   mainFinal,
  //   amountFinal,
  // });
  return amountFinal;
}

/**
 *
 * @param amount {string}
 * @param decimals {number} e.g. 6 would return 6 decimal places like 0.000000
 * @param divisorPower {number} e.g. 0 for yocto, 24 for [base], 27 for kilo, etc 
 * @param locale {string} e.g. 'en-US' or 'de-DE'
 * @returns {string} e.g. 1,000.000000
 */
export function round(amount: string, decimals = 0, divisorPower = 0, locale?: string): string {
  if (divisorPower < 0) {
    throw new Error('divisorPower must be >= 0');
  }
  const amountCleaned = amount.replaceAll('_', '');
  const divisor = Math.pow(10, divisorPower);
  const value: string = exactMath.div(amountCleaned, divisor, { returnString: true, maxDecimal: amount.length + decimals }); // ⭐ https://www.npmjs.com/package/exact-math#the-config-maxdecimal-property-usage
  // console.log(`round(${amount}, decimals = ${decimals}, divisorPower = ${divisorPower}) = ${value}`, divisor);
  const localeString = getLocaleStringToDecimals(value, decimals, locale);
  return localeString;
}

我的测试用例现在通过了。谢谢!

请参阅https://stackoverflow.com/a/68906367/，了解是什么激发了我的代码。