const mul = (a, b = 2) => a * b;

const badlyCurriedMul = curry(mul);
badlyCurriedMul(1) // 2, the curried function will use the default parameter immediately.

// And that's because mul.length is 1.
console.log(mul.length); // 1
// That's right, function length will not take default parameter into account
// so we need to specify the correct length
const curriedMul = curry(mul, 2);
curriedMul(1); // fn ...
curriedMul(1, 5); // 5
// and now it work