This works.

console.log(args.reduce((a,b)=> a*b));

You are using the reduce method wrong. The function being provided as an argument must return the value of the operation, but in your case it is console.log() which is why you are getting NaN.

This is how you are supposed to use it

function multiply(...args){
    return args.reduce((a, b) => a * b);
};

console.log(multiply(...list));

Explanation

The first argument is the accumulator and the second argument is the current value

The reduce method passes each element of the array as the second argument, whereas the first argument is initialised with the first value in the array for the first time and the result of the function from next time.

I have written the same method in a different way so it's easier to see what's going on under the hood,

function multiply(...args){
    return args.reduce(function (a,b) { console.log(a + ',' + b); return a*b; });
};

You can run the above method in the browser's developer tools and this is the output you'll get

As you can see, first 1, 2 gets logged which is the value of the accumulator and the second value in the array. Now the multiplication happens a*b which results in 1*2=2 which will be sent as the accumulator value for the next time.

Next, 2, 3 gets logged since a = 1*2 = 2 and b = 3 (third element in array)

Next 6, 4 gets logged since a = 2*3 = 6 and b = 4 (fourth element in array)

and so on.

Please refer this MDN article for more information.