if (arguments.length === 2 ){
    r = arguments[0]
    pi = arguments[1]
}else if (arguments.length === 1){
    r = arguments[0] 
    pi = 3.14
}else{
    alert('参数错误')
}

return pi * (r * r);

参数还可以加 if (typeof arguments[0] ===Number ){ ...... }