Debugging

Debugging

Introduction

Adding middleware to the fetch()–client.get(), client.post(), etc..–call hides a lot of complexity, which is good. But it also makes debugging harder. So MetroJS adds a number of tools to make debugging easier.

The most important of these are tracers.

Tracing middleware

Tracers allow you to look under the hood and see what is going on in each middleware function. Every time a middleware function is called, the client checks if any tracers are set. If so, the request tracer is called before the middleware function, and the response tracer is called after the middleware function.

A tracer function cannot alter the request or response. But you can use it to log information. Or you can set a debugger trap if a specific condition is met, e.g:

metro.trace.add('debug', {
  request: (req) => {
    if (req.searchParams.has('foo')) {
      debugger;

    }

  }
})

A tracer is an object with at most two functions, named ‘request’ and ‘response’. You don’t have to specify both of them. A tracer function doesn’t return anything. It can not change the request or response.

You can add more than one tracer. Each name must be unique. You can remove a tracer by name, or clear all tracers. Tracers are stored globally, and run on any metro client request.

There is a default tracer method included with MetroJS, called metro.trace.group. You can add it like this:

metro.trace.add('group', metro.trace.group())