Introduction

import metro from '@muze-nl/metro'

const client = metro.client('https://jsonplaceholder.typicode.com/')
  .with(metro.mw.json())

const response = await client.get('/posts/1')
console.log(response.data.title)

Metro is an HTTP client built around the Fetch API. It does not replace Fetch objects with a separate model; Metro requests and responses are proxies around standard Request and Response objects, so normal Fetch code still works. The additions are small but useful: a reusable client, middleware, a .with() method for deriving requests and responses without mutating them, and optional packages for common client-side concerns.

A Metro middleware function looks like this:

async function addHeader(req, next, context) {
  return next(req.with({
    headers: {
      'X-App': 'demo'
    }
  }))
}

Middleware can change a request before it reaches fetch(), inspect or change the response on the way back, return a mock response, throw an error, or call other Metro clients while preserving trace context. That makes it a good place for JSON handling, retries, timeouts, auth, linked-data parsing, and debugging hooks.

Start with the quickstart, then use the package structure page to decide whether the combined package or focused package imports fit your project better. If you are writing your own request behaviour, read Writing middleware.