1.循环法

function fel(n) {
    if (n  === 1 || n === 2) {
        return 1;

    }
    var n1 = 1;
    var n2 = 1;
    var fiba;

    for (var i =3;i<=n;i++) {
        fiba = n1+n2;
        n1 = n2;
        n2 =fiba;
    }
    console.log(fiba)
    return fiba;
}
fel(8)

2.递归法

function fiba(n) {
    if (n === 1 || n===2) {
        return 1;
    }else {
        return fiba(n-1) + fiba(n-2); 
    }
}