How can I convert a JavaScript BigInt value to Scientific Notation?

I would like to render a JavaScript bigint value to a string in scientific notation.

I thought of Number.toExponential() but it only works for numbers.

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

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

Intl does support Bigint:

Turns out BigInt.prototype.toLocaleString() can be used with an options for scientific-notation:

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" 

Original answer:

(This code is still useful for JS environments without Intl support or if you need more than 20 digits of precision):

Because bigint values are always integral, and because bigint.toString() will return base-10 digits without further ceremony (other than a leading - for negative values) then then a quick-and-dirty method is to take those rendered digits and insert a radix point (aka decimal dot) after the first digit and tack on the exponent at the end, and because it's a base-10 string the exponent is the same as the length of the rendered string (neat, huh?)

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"