FromJS

Find the source of every HTML character in a JavaScript app.

FromJS 2 is coming up!

FromJS observes JavaScript data flow at runtime, and helps you analyze it.
The first version of FromJS was too flaky to be useful in practice. FromJS 2 will actually work!

What is FromJS?

FromJS is a developer tool that helps you understand how a JavaScript app works.

You can select an HTML element to view where its content came from. The screenshot shows that the string "Hello" originally came from localStorage.

Other possible string sources include string literals in the app's JS code, as well as HTML served on the initial page load.

See it in action

FromJS only works on Chrome Desktop, but you can watch some demo videos.
There's also an older Backbone TodoMVC demo that may work on your device.

React TodoMVC Demo

Does it work on all websites?

Nope, most websites will break if you try to inspect them with FromJS. FromJS uses Babel to add tracking to strings, but this results in a lot of flakiness. A better approach would be to build the tracking into the JavaScript engine.
Non-trivial apps also take a long time to load and use up large amounts of memory.

Chrome Extension

Install the FromJS Chrome Extension to try FromJS yourself. The Github readme lists some example sites where FromJS works well.