javascript - 无法读取未定义的属性“createStream”

Question

【图中输入err】【1】【1】：https://i.stack.imgur.com/pelro.png

TypeError: Cannot read property 'createStream' of undefined
> this is my code: 
const rfsStream = rfs.createStream('server.log',  size: '10M',
interval: '1d', 
path: path.join(__dirname, 'public', 'logs'), });

app.use(morgan('combined', { stream: rfsStream }));