let inputArr = [40, 20, 30, 20, 10, 10, 40];

let yourFunction = (arr, countMax) => {

    if (arr.length < countMax) throw new Error('Incorrect value for argument countMax');

    array = Array.from(new Set(arr)).sort( (a,b) => { return a - b });

    return array.slice(array.length - countMax);
}

let res1 = yourFunction(inputArr, 3); //res1 is [20, 30, 40];
console.log(res1);
try {
    // get 10 maximum values
    let res2 = yourFunction(inputArr, 10);
    console.log(res2);
} catch (err) {
    // err is like “Incorrect value for argument countMax”
    console.error(err.message);
}