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https://github.com/exogen/next-fetch-har

Debug Next.js SSR requests using HTTP Archive (HAR) logs
https://github.com/exogen/next-fetch-har

debugging har http-archive nextjs react

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Debug Next.js SSR requests using HTTP Archive (HAR) logs

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README 

        
# next-fetch-har



```console

$ yarn add next-fetch-har

```



![Demo](./demo.gif)



## Motivation



When you click from page to page in a [Next.js](https://nextjs.org/) app, it’s

easy to see and debug what API calls your `getInitialProps` methods are making:

just look in your browser’s Network tab.



But what about during Server Side Rendering (SSR)? In my experience, one of the

harder aspects of debugging Next.js apps is getting visibility into what

`getInitialProps` is doing on the server – particularly what API requests and

responses the app is seeing.



Usually, this is where good logging comes in. It’s very useful to have an HTTP

client that can be instrumented with detailed logging so that you can see what

requests are being made and whether they were successful. But how much can you

reasonably log without spamming your console? All the headers? Large response

bodies? What about timing information, like how long it took to receive each

response?



Even if you connect to the [Node.js Inspector](https://nodejs.org/en/docs/guides/debugging-getting-started/),

there is no server-side equivalent to the browser’s Network tab. So what if you

had some other way to populate the Network tab with the network activity from

the server, as if those requests were made in the browser?



That’s what [node-fetch-har](https://github.com/exogen/node-fetch-har) and this

library allow you to do!



## Usage



#### Step 1: Wrap your <App>



This library exports a `withFetchHar` Higher Order Component (HOC) that you can

wrap your `` component with to enable recording of server-side Fetch API

calls.



In `_app.js`:



```js

import App from "next/app";

import { withFetchHar } from "next-fetch-har";



export default withFetchHar(App);

```



Or with a custom ``:



```js

class CustomApp extends App {

  // Your customizations...

}



export default withFetchHar(CustomApp);

```



If you haven’t exposed a global `fetch` polyfill on the server, you can supply

your `fetch` instance to the HOC:



```js

import fetch from "isomorphic-unfetch";



export default withFetchHar(App, { fetch });

```



If you’d like to enable recording only for certain pages, the HOC also works

with individual Page components:



```js

function MyPage() {

  // Your page content...

}



export default withFetchHar(MyPage);

```



#### Step 2: Use the enhanced Fetch



Instead of using a global Fetch instance, the `withFetchHar` HOC creates a

per-request instance of Fetch that logs requests as HAR entries. It adds this

Fetch instance to the `ctx` object that your pages receive in `getInitialProps`.

You should switch your calls to use this instance of Fetch.



```js

class HomePage extends React.Component {

  static async getInitialProps(ctx) {

    // Get `fetch` from `ctx`.

    const { fetch } = ctx;



    // Example of what you might do with your API...

    const response = await fetch("/api/foo");

    const body = await response.json();



    return { value: body.foo };

  }



  render() {

    // Do something with value...

  }

}

```



#### Step 3: Download the HTTP Archive



When `getInitialProps` is complete, the `withFetchHar` HOC adds the resulting

HTTP Archive (HAR) to the app’s `getInitialProps` output, and renders a download

button at the bottom of the page to access it.



![Demo](./demo.gif)



### Enable in certain environments



You probably don’t want to enable HAR logging for every request in production

(or if you do, it should probably only happen for superusers). **Remember that

HAR logs can contain sensitive information like passwords and cookie values!**



The `withFetchHar` HOC has an `enabled` option you can use to conditionally

enable HAR logging. When disabled, `ctx.fetch` will be the vanilla Fetch

instance instead of the enhanced one.



```js

withFetchHar(App, {

  // For safety, this is the default!

  enabled: process.env.NODE_ENV !== "production"

});

```



If you have multiple environments that run production builds of the app (i.e.

a staging server), you can base this check on a different condition. For

example:



```js

withFetchHar(App, {

  enabled: process.env.DEPLOY_ENV !== "production"

});

```



You can also supply a function, which will be passed the same `ctx` object

received by `getInitialProps`. You can use this to inspect `req` (or other

properties) for special superuser status or other conditions:



```js

withFetchHar(App, {

  enabled: ctx => ctx.req.user.isAdmin

});

```



## Troubleshooting



#### Some browser APIs like Headers, URLSearchParams, etc. are not found!



[node-fetch-har](https://github.com/exogen/node-fetch-har) may require access to

some of the browser APIs that Fetch uses. While [node-fetch](https://github.com/bitinn/node-fetch)

does export these, they aren’t necessarily globally available; some libraries

like [isomorphic-fetch](https://github.com/matthew-andrews/isomorphic-fetch) set

them on the `global` object for you.



You have two options:



- Make them available on `global` yourself or using a library.

- Pass them as options to `withFetchHar`, for example:



  ```js

  import { Response } from "node-fetch";



  export default withFetchHar(App, { Response });

  ```



#### What if I don’t call fetch directly in my getInitialProps?



You’ll need to find some way to pass the `ctx.fetch` instance through to your

code that needs to call it. It’s not possible to expose the enhanced HAR-enabled

Fetch instance globally because it is scoped per request: it should only capture

the requests made by each page, even if multiple are being served in parallel.



For example, if you make API calls inside Redux actions using a side-effect

solution like [redux-thunk](https://github.com/reduxjs/redux-thunk), you can

pass `ctx.fetch` to your store creation function so that it can be supplied to

any side-effect middleware.



For redux-thunk in particular, you can use its [withExtraArgument](https://github.com/reduxjs/redux-thunk#injecting-a-custom-argument)

feature to pass a custom object containing `fetch` and whatever else you like:



```js

function actionCreator() {

  return async (dispatch, getState, context) => {

    const response = await context.fetch("/api/foo");

  };

}

```



For scenarios where your store may sometimes have been created by `getInitialProps`

and other times not, you can always fall back to vanilla Fetch, since we can

only capture activity during `getInitialProps` anyway:



```js

const store = createStore(

  reducer,

  applyMiddleware(

    thunk.withExtraArgument({

      fetch: ctx ? ctx.fetch : global.fetch

    })

  )

);

```



#### How do I prevent this from disabling automatic prerendering?



Next.js’ [automatic prerendering](https://github.com/zeit/next.js#automatic-prerendering)

feature is disabled if your custom `` component defines a `getInitialProps`

method. For most advanced apps, this is often inevitable, but if this library is

the only thing causing your app to have `getInitialProps`, then you might want

to work around it.



Using the `enabled` option is not good enough to prevent the resulting ``

from having `getInitialProps`, since `ctx.fetch` needs to be defined even when

HAR logging is disabled (for your app to work whether enabled or disabled).



So, you’ll need to conditionally apply `withFetchHar` yourself and use a

fallback wherever you access `ctx.fetch`:



```js

let CustomApp = App;



if (process.env.NODE_ENV !== "production") {

  CustomApp = withFetchHar(CustomApp);

}



export default CustomApp;

```



In pages:



```js

static async getInitialProps({ fetch = global.fetch }) {

  const response = await fetch('/api/foo');

}

```



If you are only interested in recording requests for certain pages, you may

also use the `withFetchHar` only on those page components, and prerendering

should not be affected on your other pages.