An explanation of mixed type for Unknown Arity

MultiplicatorWith.js

'use strict';

const mul = (a) => {
	const result = (b) => {
		return mul(a * b);
	};
	result[Symbol.toPrimitive] = () => {
		return a;
	};
	result.valueOf = () => {
		return new Number(a).valueOf();
	};
	return result;
};

console.log(mul(1) === 1);			// false
console.log(mul(1) == 1);			// true
console.log(mul(1)(2) == 2);			// true
console.log(mul(1)(2).valueOf() === 2);		// true