javascript - 如何将 JavaScript BigInt 值转换为科学记数法？

Question

我想将 JavaScript bigint 值呈现为科学计数法中的 string。

我想到了Number.toExponential()但它只适用于 numbers。

const scientificNotation = n => parseInt(n).toExponential();

console.log(scientificNotation(prompt()));

Answer 1

Intl 确实支持 Bigint:

原来 BigInt.prototype.toLocaleString() 可以与 options 一起用于科学记数法:

const fmt /*: BigIntToLocaleStringOptions */ = {
  notation: 'scientific',
  maximumFractionDigits: 20 // The default is 3, but 20 is the maximum supported by JS according to MDN.
};

const b1 = 1234567890123456789n;

console.log( b1.toLocaleString( 'en-US', fmt ) ); // "1.234567890123456789E18" 

---

原答案:

(此代码对于不支持 Intl 的 JS 环境或者如果您需要超过 20 位的精度仍然有用):

因为 bigint 值始终是整数，并且因为 bigint.toString() 将返回以 10 为底的数字而无需进一步的仪式(除了领先的 - 对于负值)然后一个快速而肮脏的方法是获取那些呈现的数字并在第一个数字之后插入一个小数点(又名小数点)并在指数处添加结束，并且因为它是一个以 10 为底的字符串，所以指数与呈现的字符串的长度相同(整洁，是吧？)

function bigIntToExponential( value: bigint ): string {
    
    if( typeof value !== 'bigint' ) throw new Error( "Argument must be a bigint, but a " + ( typeof value ) + " was supplied." );

    //

    const isNegative = value < 0;
    if( isNegative ) value = -value; // Using the absolute value for the digits.

    const str = value.toString();
    
    const exp = str.length - 1;
    if( exp == 0 ) return ( isNegative ? "-" : '' ) + str + "e+0";

    const mantissaDigits = str.replace( /(0+)$/, '' ); // Remove any mathematically insignificant zeroes.

    // Use the single first digit for the integral part of the mantissa, and all following digits for the fractional part (if any).
    let mantissa = mantissaDigits.charAt( 0 );
    if( mantissaDigits.length > 1 ) {
        mantissa += '.' + mantissaDigits.substring( 1 );
    }

    return ( isNegative ? "-" : '' ) + mantissa + "e+" + exp.toString();
}

console.log( bigIntToExponential( 1n ) );    // "1e+0"
console.log( bigIntToExponential( 10n ) );   // "1e+1"
console.log( bigIntToExponential( 100n ) );  // "1e+2"
console.log( bigIntToExponential( 1000n ) ); // "1e+3"
console.log( bigIntToExponential( 10000n ) ); // "1e+4" 
console.log( bigIntToExponential( 1003n ) ); // "1.003e+3" 
console.log( bigIntToExponential( 10000000003000000n) ); // "1.0000000003e+16" 
console.log( bigIntToExponential( 1234567890123456789n ) ); // "1.234567890123456789e+18" 
console.log( bigIntToExponential( 12345678901234567898765432109876543210n ) ); // "1.234567890123456789876543210987654321e+37" 

console.log( bigIntToExponential( -1n ) );    // "-1e+0"
console.log( bigIntToExponential( -10n ) );   // "-1e+1"
console.log( bigIntToExponential( -100n ) );  // "-1e+2"
console.log( bigIntToExponential( -1000n ) ); // "-1e+3"