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Last active September 27, 2022 00:41
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Web frontend code coverage

Every line of code we write today will end up as someone’s legacy code.

Everything that you write is going to disappears some day

RUM Code coverage, chose a 1h timeframe (per day) where 1% traffic run that the test

JS coverage

FES javascript instrimentation visualization

Create babel plugin that inject in code with visitors for functions and control flow (loops and switchs): Function, IfStatement, (Terminatorless, SwitchCase, Loop, TryStatement)

Or as standalone tool:


CSS coverage

Aka unused CSS RUM


Can't support easily CSSOM changes

Parse + generate result client side (use web worker):

  1. get CSSOM (for external and inlined stylesheets) from DOM (see step 4)
  2. download CSS + map
  3. parse stylesheet to CSSOM: ruleset, selectors, values, etc. (for results from step 1 and 2)
  4. find which rules/properties has been ignored by the browser (compare results from step 3)
  5. apply it to a virtual DOM
  6. apply document updates to the virtual DOM based on MutationObserver (for added or removed external and inlined style sheets too, see steps 1, 2 and 3)
  7. apply source map
  8. generate LCOV report + send to server (navigator.sendBeacon()) with generated version (git commit hash)

Server side:

  1. merge reports
  2. apply later changes (commits diffs, in that case, the final report should used carefully)

Based on Webdev tools code (Firefox / Chrome):

An other method to detect ruleset usage is to inject a rarely used property with a custom value and get style with void selector (could be enabled and disabled synchronously): voice-family: "my custom value" (could be also font-family, content, quotes but could add layout impact). This method could be hard to implement if the stylesheet use attribute selectors (should detect which element or its parent has attribute change and list which attribute changes to "listen") It could also use a tracker image with some properties (CSS background or content property)) to direct tracking element but generate too much traffic (each document elements and pseudo element call a beacon).

Some tips, tricks, etc.:

Libs and tools:

Similar tools

Code coverage in IDE

In Intellij IDEA/WebStorm/PHPStorm:

In Visual Studio Code:

In Visual Studio:


Other data format

Aka custom data format

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