长轮询的实现代码

{
            let a = function*() {
                yield new Promise((resolve, reject) => {
                    setTimeout(() => {
                        resolve({
                            code: 0
                        });
                    }, 500);
                });
            };
            let b = function() {
                let g = a();
                let s = g.next();
                s.value.then(function(d) {
                    if (d.code != 0) {
                        setTimeout(() => {
                            console.log('wait');
                            b();
                        }, 500);
                    } else {
                        console.log('code为0，成功');
                    }
                });
            };
            b();
        }