Explore DevTools Network panel

Jeff Posnick
Jeff Posnick

This codelab walks you through the process of interpreting all of the network traffic for a more complex sample application. At the end of the exercise, you'll have the skills you need to figure out what your own web application's loading, and when each of the requests are being made.

The screenshots and instructions in this codelab assume that you're using Chrome. Each browser has its own DevTools experience, which might not match what you see in this codelab.

Navigate to the Network panel to see the network traffic for the demo application.

  • To preview the site, mouse over the editor, press the App button, then the Show button.
  1. Open DevTools by pressing CMD + OPTION + i / CTRL + SHIFT + i.
  2. Select the Network tab.
  3. Reload the page to see the network traffic.

You should see the DevTools interface open up, in either a new window or as a sidebar in your existing window. In the DevTools, select the Network tab to open the Network panel. The Network panel shows all the assets loaded as a result of your initial navigation:

Chrome DevTools' network panel.

The actual columns you see in the Network panel may be different; the screenshot shows a simplified view with everything but the Name, Type, and Waterfall columns hidden.

How to interpret the entries

Each row of recorded network traffic represents a single request and response pair.

The first row, with type document, is the initial navigation request for the web app's HTML. This is the "source" for the waterfall; each of the subsequent requests for additional assets (known as subresources of the main document) flow from this original source.

The second and third rows, showing a CSS stylesheet and a script subresource being loaded, are dependent requests that were initiated by the main document.

Looking at when those requests are made, the waterfall diagram shows that they're not started until very late in the process of responding to the navigation request.

Taken together, the requests for the HTML document along with some CSS and JavaScript are the set of requests needed to display the full page during the initial navigation.

Create some additional runtime requests

With the Network panel still open and recording, it's time to simulate something common for a lot of web apps: additional API requests used to add more data to the page after the initial navigation is complete.

Trigger these additional requests by clicking the Find Me button, and then Allowing the site to access your current location:

The allow location permission prompt.

You could also deny Geolocation permission, in which case the web app will fall back to a default location.

Once the web app has a location to work with, clicking on the Find Nearby Wikipedia Entries button results in several additional Network requests. You should see something like this:


Interpret the new entries

As before, each row of recorded network traffic represents a single request and response pair.

The first row of the new entries represents a request with a type of fetch, which corresponds to the way the web app requests data from the Wikipedia API.

The following rows all are images (png or jpeg) associated with the Wikipedia entries. Although it's a little hard to see from the screenshot, their entries in the Waterfall column directly flow from the API response.

For all of these additional requests, the when is going to vary based on how long you've had the page open before you click on Find Nearby Wikipedia Entries. Most important here is that the when is disconnected from the initial navigation request. You can tell this from the large gap that exists in the Waterfall display, representing a period of time that passed in between the initial loading and when the Wikipedia API request is made.

Requests made after a gap of time following a navigation fall into the category of "runtime requests," as opposed to the initial set of requests used to display the page when you first navigated to it.

Summing things up

Having gone through the steps in this codelab, you're now familiar with the tools you can use to analyze what any web application loads.

The Network panel helps you answer the question of what's being loaded, via the URLs in the Name column and the data in the Type column, along with when it's being loaded, via the waterfall display.

You've also seen that requests made by a web page can (usually) be grouped into one of two categories:

  1. Initial requests, made right after navigating to a new page, for the HTML, JavaScript, CSS (and potentially other assets).
  2. Runtime requests made as a result of user interaction with the page. This can often start with a request to an API, and then flow into several follow-up requests based on the API data retrieved.