# 错误控制

Thanks to co, error handling is much easier in koa. You can finally use try/catch!

## 抓住下游错误

In Express, you caught errors by adding an middleware with a (err, req, res, next) signature as one of the last middleware. In contrast, in koa you add a middleware that does try { yield next } as one of the first middleware. It is also recommended that you emit an error on the application itself for centralized error reporting, retaining the default behaviour in Koa.

app.use(function *(next) {
try {
yield next;
} catch (err) {
this.status = err.status || 500;
this.body = err.message;
this.app.emit('error', err, this);
}
});

app.use(function *(next) {
throw new Error('some error');
})


This opens up a bunch of new possibilities:

• As you go down the stack, you can change error handlers very easily.
• You can handle errors in portions of your code much more easily.