Prímteszt

let pt = [2], i, prim
for(i=3;i<1000;i+=2) {
    prim=true ;
    pt.forEach(v => {
        if ( !(i%v) ) prim=false
    })
    if (prim) pt.push(i)
}
let ph = new Set(pt)
console.log(ph.has(998));