Double all elements of array without array helper methods

double_array_without_built_in_helpers.js

const numbers = [1, 2, 3];

function double(num) {
    let [firstNumber, ...restNumbers] = num;
    if (firstNumber) {
        return [2 * firstNumber, ...(double(restNumbers))];
    } else {
        return [];   
    }
}

double(numbers);
/// [ 2, 4, 6]