let a=0;
let b=1;
let f=0;



for (let n=0; n<50; n++) {

  switch(n) {
    case 0: f=0; break;
    case 1: f=1; break;
    default: {
      f=a+b;
      a=b;
      b=f;

    }
  }

  console.log("n="+n+" f="+f);
}