This implementation is useless for more than 47 digits.

gistfile1.js

function* calculatePi(){
    let [q, r, t, k, n, l] = [1, 0, 1, 1, 3, 3];
    while(true){
        if(4 * q + r - t < n * t){
            yield n
            nr = 10 * (r - n * t)
            n = ((10 *(3 * q + r)) % t)  - 10 * n
            q *= 10
            r = nr
        }else{
            nr = (2*q+r)*l
            nn = (q*(7*k)+2+(r*l))%(t*l)
            q  *= k
            t  *= l
            l  += 2
            k += 1
            n  = nn
            r  = nr
        }
    }
}