ex06.1_function_type_parameters.js

function getMeanTemp(temperatures) {
    if (!(temperatures instanceof Array)) {
        return NaN;
    }
    let sum = 0;
    for (const element of temperatures) {
        sum += element;
    }
    return sum / temperatures.length;
}

console.log(getMeanTemp(10));       // -> NaN
console.log(getMeanTemp([10, 30])); // -> 20