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Debug Workers

Debugging is a critical part of developing a new application — whether running code in the initial stages of development, or trying to understand an issue occurring in production. In this article, you will learn how to effectively debug your Workers application, as well as review some code samples to help you get started:

​​ Local testing with wrangler dev

The wrangler dev command starts a local server for developing your Worker. wrangler dev allows you to get feedback quickly while iterating by exposing logs on localhost and to experiment without deploying to production. wrangler dev can significantly reduce the time it takes to test and debug new features.

wrangler dev will run the preview of the Worker directly on your local machine using the open source Cloudflare Workers runtime, workerd and the Miniflare simulator.

In addition to testing Workers locally with wrangler dev, the use of Miniflare allows you to test other Developer Platform products, such as R2, KV, D1, and Durable Objects.

​​ Local testing against remote resources

wrangler dev runs locally by default. To deploy your application to the edge preview service, and make it available for access on localhost, run wrangler dev --remote:

$ wrangler dev
Built successfully, built project size is 27 KiB.
Using namespace for Workers Site "__app-workers_sites_assets_preview"
Uploading site files
Listening on http://localhost:8787
[2020-05-28 10:42:33] GET HTTP/1.1 200 OK
[2020-05-28 10:42:35] GET HTTP/1.1 200 OK
[2020-05-28 10:42:36] GET HTTP/1.1 200 OK

In the output above, you can begin to see log lines for the URLs being requested locally.

​​ DevTools

Wrangler supports using the Chrome DevTools to view logs/sources, set breakpoints, and profile CPU/memory usage. With wrangler dev running, press the d key in your terminal to open a DevTools session connected to your Worker.

​​ Customize wrangler dev

You can customize how wrangler dev works to fit your needs. Refer to the wrangler dev documentation for available configuration options.

​​ Livestream logs from deployed Workers with wrangler tail

To get more insight into currently deployed Workers, run wrangler tail. wrangler tail starts a session to livestream logs from a deployed Worker.