The first time-traveling debugger for Svelte applications.
DeLorean is a debugging tool for Svelte developers. It records snapshots when a target applications's state changes and allows users to jump to any previously recorded state. It also displays the names and values of all variables in your application at the time of the snapshot alongside the state jump button.
All stateful data is easily accessible from within DeLorean’s Chrome Developer Tools panel. The name of each component and the names of all variables that that component contains, alongside their values, are displayed. As you make changes to your app, each component’s state at the time of the state change is stored in a snapshot and cached. These snapshots are then displayed in the Dev Tools panel, and also allow for the next feature:
In addition to displaying component and variable information, DeLorean resets your application’s state to the values it contained at any point since DeLorean was connected to your application. This allows for step-by-step examination of state change sequences within an application, easing the challenge of tracking state changes over time and reducing the need to use console.log.
As DeLorean renders an app in its previous state, a user may want to interact with the application in a different way than before. Apps remain fully functional while being tested with DeLorean, so any changes the user makes will simply create a new timeline that is now tracked in the Dev Tools panel.
If you're interested in learning more about how DeLorean works, feel free to fork and clone this repo! Otherwise, just download the chrome_extension folder and save it somewhere on your computer.
Then navigate to Chrome's extensions page. Ensure you are in developer mode by clicking the 'developer mode' switch in the top-right corner of the page. Click on 'load unpacked', and select the chrome_extension folder downloaded earlier. Open up your DevTools panel, and check to make sure DeLorean is available in the dropdown menu of the navbar!
Importantly, this debugging tool can only operate on Svelte applications being run in dev mode. Without dev mode enabled, Svelte's compiler strips some internal functionality in order to reduce overall bundle size. DeLorean requires that functionality, so ensure dev mode is enabled when you run the application to be tested.
At this time, DeLorean searches the tested application for an id of "root" to find Svelte components to test. An example format for an html page and its main svelte component:
index.html:
<head></head>
<body>
<div id="root" />
</body>
main.js:
import SvelteApp from './SvelteApp.svelte';
new SvelteApp({
target: document.getElementById('root'),
});
If your application attaches to something other than an element with the id "root", DeLorean won't be able to find it.
This extension's permissions are scoped to localhost:<PORT> urls only, so make sure to run your application on a localhost port in order to use DeLorean.
Once your app is up and running, open the Dev Tools panel and select DeLorean from the dropdown in the navbar. Then click Connect, and you should see your application's intial state.
Have fun! DeLorean tracks every stateful update and reflects it in real time in the Dev Tool panel. If you click on the state buttons that appear in the Dev Tool panel, you will see your application's state at that moment reflected in the application, as well as each component's variables with their names and values displayed in the devtool.
Changing state after clicking on a previous state button will lead to the creation of a memory stack. This erases the old changes made to your application's state and allows you to explore a different sequence of state changes on your application, without needing to reresh or restart your application.
[] Did you install DeLorean?
[] Try refreshing the extension from the Chrome extensions page
[] Is your application running in dev mode?
[] Is your Svelte component attached to an element with the id of "root"?
[] Are you running your app from something other than a localhost url?
[] Was your app developed with SvelteKit? At this time, DeLorean is untested with SvelteKit applications, and there are likely to be implementation issues. If you want DeLorean to work with SveleteKit now, feel free to contribute to the project!
[] DeLorean receives initial state after connecting for the first time. If you've closed and reopened the Dev Tools panel, just click Connect and change some state in your application and DeLorean will start to track it, although only from the point of second connection onwards.
DeLorean is fully open-source, and we would love to get more brains working on the project. If you'd like to fix bugs or add a new feature, fork the repo, make some changes and submit a pull request. Our team will review your changes and work to integrate functional code.
We have a number of features in our backlog to implement, including consistent SvelteKit integration, creation of an ignore list to stop tracking certain variables, and persistent storage, so that state history can be saved even after closing out of the extension.
If you have a feature request in mind, please submit an issue so our team can determine if it is an appropriate feature for DeLorean.
Visit the DeLorean website
Read more at Medium - Time Travel Debugging in Svelte with DeLorean
Special thanks to RedHatter's Svelte Dev Tools for providing examples of how Svelte's dev mode listeners can be utilized.
Albert Han - @alberthan1
Aram Krakirian - @aramkrakirian
Erick Maese - @ErickMaese
Trevor Leung - @trevleung
Sam VanTassel - @SamVanTassel