correction

EX1

let max = arr.reduce(function(a,c) {  return (a.length> c.length)?a:c;});

EX2

EX3

let calcul = function (rayon) {

   const PI = 3.14;

   let perimetre = 2*PI*rayon,

    volume = 4/3*PI*Math.pow(rayon,3),

    diametre = rayon+rayon;

   return {perimetre,volume,diametre};

};

let {perimetre,volume} = calcul(val);

EX4

function foo ( ...args ) {

   args = args.filter( v => v % 2 == 0);

   console.log( args );

}

EX5

console.log(amis.reduce(function(prev, curr) {

 return [...prev, ...curr.prenoms];

},[]););