Convert a number to a different base (e.g., from hex to decimal)

changeBase.js

//Convert a number to a different base (e.g., from hex to decimal).
//Returns a string representation of the number, or NaN if `num` does not represent a number of the specified base.
function changeBase(num, fromRadix, toRadix){
	if(!(1*fromRadix) || fromRadix%1 !== 0 || fromRadix < 2 || fromRadix > 36){
		throw new RangeError(`'fromRadix' must be an integer between 2 and 36, inclusive.`);
	}
	if(!(1*toRadix) || toRadix%1 !== 0 || toRadix < 2 || toRadix > 36){
		throw new RangeError(`'toRadix' must be an integer between 2 and 36, inclusive.`);
	}
	
	num = (""+num).toUpperCase();
	
	//convert the number to decimal (BigInt)
	let symbols = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ".slice(0, fromRadix),
		rxp = new RegExp(`^([${symbols}]*)(?:\\.([${symbols}]*))?$`),
		matches = rxp.exec(num);
	if(!matches || num == ".") return NaN;	//`num` is not a number of the specified base
	let integer = matches[1] || "0",
		fraction = matches[2];
	integer = parseBigInt(integer, fromRadix);
	if(fraction) fraction = parseBigInt(fraction, fromRadix);
	
	//convert the decimal number to desired base
	return integer.toString(toRadix) + (fraction ? "."+fraction.toString(toRadix) : "");
}

function parseBigInt(string, radix){
	if(!(1*radix) || radix%1 !== 0 || radix < 2 || radix > 36){
		throw new RangeError(`'radix' must be an integer between 2 and 36, inclusive.`);
	}
	string = ""+string;
	let symbols = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ".slice(0, radix);
	if(!RegExp(`^[${symbols}]+$`, "i").test(string)) return NaN;	//`string` does not represent a number of the specified base
	radix = BigInt(radix);
	return [...string].reduce((result, currentDigit)=>(result*radix + BigInt(symbols.indexOf(currentDigit))), 0n);
}