1

function sum(...rest) {
       var s = 0;
       rest.forEach(function(element,index,self){
           s += element;
       });
       return s;
}

2

if(typeof r !== 'number'){
    console.log('半径不是数字');
    return;
}
if(typeof pi !== 'number' || isNaN(pi) || arguments.length<2   ){
    pi=3.14;
}
return pi*r*r;

3

//return a>b?a:b;

if ( a > b ) {
    return a;
} else {
    return b;
}